System and method for nanoparticle and nanoagglomerate fluidization

ABSTRACT

With the coupling of an external field and aeration (or a flow of another gas), nanoparticles can be smoothly and vigorously fluidized. Multiple external fields and/or pre-treatment may be employed with the fluidizing gas: sieving, magnetic assistance, vibration, acoustic/sound or rotational/centrifugal forces. Any of these forces, either alone or in combination, when coupled with a fluidizing medium, provide excellent means for achieving homogenous nanofluidization. The additional force(s) help to break channels as well as provide enough energy to disrupt the strong interparticle forces, thereby establishing an advantageous agglomerate size distribution. Enhanced fluidization is reflected by at least one of the following performance-related attributes: reduced levels of bubbles within the fluidized system, reduced gas bypass relative to the fluidized bed, smooth fluidization behavior, reduced elutriation, a high level of bed expansion, reduced gas velocity levels to achieve desired fluidization performance, and/or enhanced control of agglomerate size/distribution. The fluidized nanoparticles may be coated, surface-treated and/or surface-modified in the fluidized state. In addition, the fluidized nanoparticles may participate in a reaction, either as a reactant or a catalyst, while in the fluidized state.

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of the following co-pending,commonly assigned provisional patent applications: (i)“Vibrofluidization and Magnetically Assisted Fluidization ofNanoparticles,” filed on Jul. 29, 2003 and assigned Ser. No. 60/490,912,and (ii) “Nanoparticle and Nanoagglomerate Fluidization System andMethod,” filed on May 4, 2004 and assigned Ser. No. 60/568,131. Thecontents of each of the foregoing provisional patent applications areincorporated herein by reference.

BACKGROUND OF THE DISCLOSURE

1. Technical Field

The present disclosure relates to system(s) and method(s)/process(es)for fluidizing nanoparticles and nanoagglomerates. More particularly,the present disclosure is directed to systems and methods/processes forfluidizing nanoparticles and nanoagglomerates utilizing a fluidizing gaswith one or more external forces, e.g., a vibration force, a magneticforce, an acoustic force, a rotational force and combinations thereof.Advantageous results are achieved, at least in part, by establishing adesired nanoparticle/nanoagglomerate particle size distribution withinthe system and substantially maintaining such distribution as the systemachieves and maintains a fluidized state.

2. Background of Related Art

Fluidization is a widely used process in several industries to achievecontinuous powder handling ability, particle mixing, and desirablelevels of solid-gas contact. By definition, gas fluidization is aprocess in which solid particles are transformed into a fluid-like statethrough suspension in a gas. Gas fluidization is one of the besttechniques available to disperse and process powders belonging to theGeldart group A and B classifications. Fluidization processes can beused to achieve high heat and mass transfer and reaction rates. Gasfluidization of small solid particles has been widely used in a varietyof industrial applications because of its unusual capability ofcontinuous powder handling, good mixing, large gas-solid contact areaand high rates of heat and mass transfer.

Extensive research has been done in the area of gas fluidization, andthe fluidization behavior of classical powders in the size range of 30to 1000 μm (Geldart group A and B powders) is relatively wellunderstood. However, the fluidization behavior of ultrafine particles,including nanoparticles which are in the extreme low end of Group Cparticles (<20 microns) in Geldart's Classification of Powders, is muchmore complex and has received relatively little attention in theliterature. Nanoparticles are difficult to fluidize due to their stronginterparticle forces. A bed of nanosized silica, for example, willexhibit plug formation, channeling, and/or spouting in a conventionalfluidized bed. As far as is known, fluidization of nanoparticles (whichare three orders of magnitude smaller than traditional group C powders)has heretofore been extremely difficult, if not impossible, toeffectively achieve.

At least in part based on their very small primary particle size andvery large surface area per unit mass, nanostructured materials areeffective for the manufacture of drugs, cosmetics, foods, plastics,catalysts, high-strength or corrosion resistant materials, energetic andbio materials, and in mechatronics and micro-electro-mechanical systems(MEMS). Based on such uses, processing technologies which can handlelarge quantities of nanosized particles, e.g., mixing, transporting,modifying the surface properties (coating) and downstream processing ofnanoparticles to form nano-composites, are desirable. But beforeprocessing of nanostructured materials can take place, the nanosizedparticles have to be well dispersed.

Strong interparticle forces exist between nanoparticles, such as van derWaals, electrostatic and moisture-induced surface tension forces. Basedon such forces, nanoparticles are found to be in the form of large-sizedagglomerates (rather than as individual nano-sized particles) whenpacked together in a gaseous medium. Hence, gas fluidization ofnanoparticles generally refers to the fluidization of nanoparticleagglomerates.

It is generally possible to fluidize nanoparticles as relatively largeagglomerates when the gas velocity exceeds the expected minimumfluidization velocity of the agglomerates. However, there tends to besignificant powder loss and non-uniform fluidization behavior. Inaddition, large agglomerates can form near the distributor. Thus, thereremains a need for a fluidization process that minimizes or avoidspowder loss and accomplishes a smoother, more controlled fluidizationwith good mixing.

Previous studies of gas fluidization of nanoparticle agglomerates havefound that the minimum fluidization velocity is relatively high (aboutseveral orders of magnitude higher than the minimum fluidizationvelocity of primary nanoparticles). The size of the fluidizednanoparticle agglomerates is typically from about 100 to 700 μm, whilethe primary particle size ranges from 7 to 500 nm. A typicalnanoparticle agglomerate size distribution (by weight percentage) for acommercially available product (Aerosil® R972 silica; Degussa;Dusseldorf, Germany) is shown in FIG. 1. The data reflected in FIG. 1was generated by: (i) randomly sampling a storage bag of commerciallyavailable R972 silica (20.0 g), (ii) sieving the sample using ten (10)different sieve sizes and measuring the weight retained on each suchsieve, (iii) recording sieve size and particle weight, and (iv)calculating weight percentage for each sieve and plotting results. Asshown on FIG. 1, a typical agglomerate size distribution for acommercially available nanoparticles products is widely dispersed andincludes a significant weight percentage at larger agglomerate sizes.

For some nanoparticles, very smooth fluidization occurs with extremelyhigh bed expansion, practically no bubbles are observed, and thevelocity as a function of voidage around the fluidized agglomeratesobeys a modified Richardson-Zaki equation. This type of fluidization ofnanoparticle agglomerates has been termed agglomerate particulatefluidization (APF) by Wang et al [See, Wang et al., Fluidization andagglomerate structure of SiO ₂ nanoparticles, Powder Technology, 124(2002) 152-159.8]. For other nanoparticles, fluidization results in avery limited bed expansion, and large bubbles rise up very quicklythrough the bed. This type of fluidization has been termed agglomeratebubbling fluidization (ABF) by Wang et al. However, even for thehomogeneously fluidized nanoparticles, relatively large powderelutriation occurs at the high gas velocities required to fluidize thenanoagglomerates. This loss of particles may hinder the applicability offluidization of nanoparticle agglomerates in industrial processes.

In addition to conventional gravity-driven fluidization, nanoparticleagglomerates can also be fluidized in a rotating or centrifugalfluidized bed [See, Matsuda et al., Particle and bubble behavior inultrafine particle fluidization with high G, Fluidization X, Eng. Found,2001, 501-508; Matsuda et al., Modeling for size reduction ofagglomerates in nanoparticle fluidization, AIChE 2002 Annual Meeting,Nov. 3-8, 2002, Indianapolis, Ind., 138e], where the centrifugal forceacting on the agglomerates can be set much higher than gravity.

A number of studies dealing with modeling and numerical simulation ofthe fluidization of nanoparticle agglomerates can be found in theliterature. These models are based either on force or energy balancesaround individual agglomerates, the use of the Richardson-Zaki equation,or a combination of the Richardson-Zaki equation with fractal analysisfor APF fluidization, or a modified kinetic theory. Recently, someapplications of nanoparticle agglomerate fluidization were investigated,including the production of carbon nanotubes, and its application tophotocatalytic NO_(x) treatment. However, very little experimental dataon the fluidization characteristics and differences between APF and ABFnanoparticles, such as minimum fluidization velocity, agglomerate size,hysteresis effects, and the effect of nanoparticle material properties,are available.

Sound waves, in combination with vibration, have been used to increasefluidization quality in cohesive powders whose sizes range fromsubmicron to 20 microns. Also, vibration combined with gas flow has beenused to successfully fluidize particles of smaller size, such asnanoparticles. However, notwithstanding the benefits associated withthese known fluidizing techniques, often a dense immobile phase forms ata bottom of a fluidizing bed.

U.S. Pat. No. 4,720,025 to Tatevosian discloses a technique thatutilizes an alternating magnetic field along with magnetic particles toloosen up material at the bottom of a hopper for feeding into a certainoperation. However, the disclosed technique does not include looseningup cohesive materials for application in a fluidized bed. Similarly,U.S. Pat. No. 6,471,096 to Dave discloses the use of alternatingmagnetic field along with permanent magnets to produce controllabledischarge of cohesive powders from a container, but does not provide forfluidization of nano-powders. U.S. Pat. No. 3,848,363 to Lovness et al.discloses the use of magnetic force to move particles in a predeterminedarea, but again does not provide for any application to fluidization.

The idea of using a magnetofluidized bed was proposed in 1960 byFillipov [see, M. V. Filippov, The effect of a magnetic field on aferromagnetic particle suspension bed, Prik. Magnit. Lat. SSR, 12 (1960)215] and became popular as a means of suppressing bubbles in gasfluidized beds for a variety of industrial applications [see, R. E.Rosensweig, Process concepts using field stabilized two-phase flow, J.of Electrostatics, 34 (1995)163-187]. Generally, the particles to befluidized were either magnetic particles or a mixture of magnetic andnon-magnetic particles, and the magnetic field was usually generated byDC current [see; V. L. Ganzha, S. C. Saxena, Heat-transfercharacteristics of magnetofluidized beds of pure and admixtures ofmagnetic and nonmagnetic particles, Int. Journal of Heat Mass Transfer,41(1998) 209-218; J. Arnaldos, J. Casal, A. Lucas, L. Puigjamer,Magnetically stabilized fluidization: modeling and application tomixtures, Powder Technology, 44(1985) 57-6224; W. Y. Wu, A. Navada, S.C. Saxena, Hydrodynamic characteristics of a magnetically stabilized airfluidized bed of an admixture of magnetic and non-magnetic particles,Powder Technology, 90(1997) 39-46; W. Y. Wu, K. L. Smith, S. C. Saxena,Rheology of a magnetically stabilized bed consisting of mixtures ofmagnetic and non-magnetic particles, Powder Technology, 91(1997)181-187; X. Lu, H. Li, Fluidization of CaCo ₃ and Fe ₂O₃ particlemixtures in a transverse rotating magnetic field, Powder Technology,107(2000) 66-78], causing magnetic particles to form chains along thefield. For example, Arnaldos et al [see, J. Arnaldos, J. Casal, A.Lucas, L. Puigjamer, Magnetically stabilized fluidization: modeling andapplication to mixtures, Powder Technology, 44(1985) 57-6224] studiedthe fluidization behavior of a mixture of magnetic and non-magneticparticles of several hundred microns in size, such as sinterednickel-silica, steel-copper and steel-silica particles. The fluidizationof larger particle mixtures of millimeter size (Geldart group Dparticles), such as iron-copper shot of 0.935 to 1.416 mm in diameter isdescribed in [W. Y. Wu, A. Navada, S. C. Saxena, Hydrodynamiccharacteristics of a magnetically stabilized air fluidized bed of anadmixture of magnetic and non-magnetic particles, Powder Technology,90(1997) 39-46] and [W. Y. Wu, K. L. Smith, S. C. Saxena, Rheology of amagnetically stabilized bed consisting of mixtures of magnetic andnon-magnetic particles, Powder Technology, 91(1997) 181-187], and Lu etal [X. Lu, H. Li, Fluidization of CaCo ₃ and Fe ₂ O ₃ particle mixturesin a transverse rotating magnetic field, Powder Technology, 107(2000)66-78] studied the fluidization of very fine (Geldart group C) particlemixtures of CaCO₃—Fe₂O₃ in a transverse rotating magnetic field.However, in all of these studies, the magnetic particles were fluidizedalong with the non-magnetic particles.

Further, from studies at New Jersey Institute of Technology (NJIT), ithas been shown that a magnetically assisted impaction coating (MAIC)process may be an effective method for providing the extra force neededto break up the dense phase or layer of particles. The MAIC process hasbeen successfully used as a dry coating method. The MAIC processutilizes an oscillating magnetic field to accelerate magnetic particlesthereby providing collisions between particles and the walls of theapparatus. Each of the foregoing techniques are directed to the use of amagnetic field with magnets for accomplishing certain processes, butnone of the techniques are directed to fluidization of extreme Geldart Cparticles, in particular, nano-powders.

At a low sound frequency, typically from 50 to 400 Hz, and a high soundpressure level, typically above 110 dB, sound waves have been shown toimprove the fluidization of fine particles, which otherwise showedintense channeling or slugging rather than fluidization [Morse, Sonicenergy in granular solid fluidization, Ind. Eng. Chem., 47 (6) (1955)1170-1175]. Standing waves are generated in the experimental column andat a fixed sound pressure level, sound assisted fluidization can onlyoccur within a certain range of low sound frequencies. Channeling hasbeen found above and below this frequency range [Russo et al., Theinfluence of the frequency of acoustic waves on sound-assistedfluidization of beds offine particles, Powder Technology, 82 (1995)219-230]. At the natural frequency of the bed of micron sized particles,high intensity sound waves have been found to lead to reductions in boththe minimum bubbling velocity and the minimum fluidization velocities[Levy et al., Effect of an acoustic field on bubbling in a gas fluidizedbed, Powder Technology, 90 (1997) 53-57]. The literature also shows thatan increase in sound pressure level may also yield a decrease in bedexpansion, an increase in bubble frequency and an increase in bubblesize, and that high intensity sound can also effectively reduce theelutriation of fine particles [Chirone et al., Bubbling fluidization ofa cohesive powder in an acoustic field, Fluidization VII, 1992,545-553]. To date, the reported research has been directed tosound-assisted fluidization of micron or sub-micron sized particles. Noresults have been reported on the effects of sound on the fluidizationof nanoparticle agglomerates.

Thus, despite efforts to date, a need remains for systems andmethods/processes that provide for effective fluidization ofnanoparticles. A further need remains for systems and processes thatuniformly fluidize a bed of nanoparticles. Also needed are systems andprocesses for nanoparticle fluidization that function without forming adense layer of agglomerates. Additionally, fluidization systems andprocesses that minimize powder loss while fluidizing nanoparticles areneeded. It is a further need to determine characteristics ofnanoparticle agglomerates and to use such characteristics in enhancingfluidization effectiveness.

SUMMARY OF THE DISCLOSURE

The present disclosure provides an improved system and method/processfor fluidizing nanoparticles and nanoagglomerates that includes exposingnanoparticles and nanoagglomerates to a combined flow of fluidizing gasand at least one additional force. According to exemplary embodiments ofthe present disclosure, the additional force may be supplied from avariety of sources and may take a variety of forms, e.g., a vibrationforce, a magnetic force, an acoustic force, a rotational/centrifugalforce and/or a combination thereof. The disclosed system and methodutilizes a fluidizing gas (e.g., air, N₂, He, Ar, O₂ and/or combinationsthereof or other fluidizing gas or gases) that may be combined with anappropriate amount of magnetic energy, mechanical energy, acousticenergy and/or rotational/centrifugal energy to enhance fluidization bydisrupting interparticle forces. The nanoparticles/nanoagglomeratestreated according to the disclosed system/method can form highly porousagglomerates in the size range of approximately 200-400 microns.

Enhanced fluidization of nanoagglomerate/nanoparticles systems isachieved according to the systems and methods/processes of the presentdisclosure, at least in part, by establishing a desirednanoparticles/nanoagglomerates particle size distribution within thesystem and substantially maintaining such distribution as the systemachieves and maintains a fluidized state. According to exemplaryembodiments of the present disclosure, a desired particle sizedistribution is established by introducing an external energy stimulusat a level effective to overcome the inter-particle forces associatedwith nanoparticles/nanoagglomerates systems and to thereby shift theparticle size distribution into a range that supports and/or evidencesenhanced fluidization. Alternatively, a desired particle sizedistribution may be effected through a pre-treatment step, e.g., asieving step.

As used herein, “enhanced fluidization” is reflected by at least one ofthe following performance-related attributes: reduced levels of bubbleswithin the fluidized system, reduced gas bypass relative to thefluidized bed, smooth fluidization behavior, reduced elutriation, a highlevel of bed expansion, reduced gas velocity levels to achieve desiredfluidization performance, and/or enhanced control of agglomeratesize/distribution.

According to the present disclosure, modification of an initial particlesize distribution (e.g., an “as received” particle size distribution) toa desired particle size distribution range allows the disclosedfluidization system to achieve and maintain desired fluidizationconditions. Through introduction of an external energy source and/or apre-treatment step, as described in greater detail herein, the disclosedfluidization system advantageously establishes a state of dynamicequilibrium, wherein nanoagglomerates are formed, broken and randomlyreformed, in an expanded fluidized bed. The dynamic equilibriumestablished according to the disclosed system/method offers manyadvantages, including facilitating substantially homogenous coatingand/or treatment of nanoparticles/nanoagglomerates. Exemplaryfluidization apparatus according to the present disclosure includes agas supply source and at least one energy source for generating andsupplying one or more of the energies disclosed herein, e.g., avibrating source, a source for inducing a magnetic field, an acousticsource, and/or a source of centrifugal and/or rotational force. Otherfeatures that may be associated with the fluidization apparatus of thepresent disclosure include a gauge for measuring gas flow, afluidization chamber, a distributor, gas dispersion elements (e.g.,glass beads), filter(s), viewing device(s) and/or a vent.

According to the present disclosure, advantageous results are achievedin fluidizing nanoparticles and nanoagglomerates across a broad range ofapplications, e.g., applications that involve the manufacture of drugs,cosmetics, foods, plastics, catalysts, high-strength or corrosionresistant materials, energetic and bio materials, and in mechatronicsand micro-electro-mechanical systems. More particularly, effectivedispersion of nanoparticles and nanoagglomerates is achieved accordingto the present disclosure, thereby facilitating a host ofnanoparticle-related processing regimens, e.g., mixing, transporting,surface property modifications (e.g., coating), and/or downstreamprocessing to form nano-composites. In particular, by combining orcoupling the flow of a fluidizing gas with one or more external forces,the combined effect is advantageously sufficient to reliably andeffectively fluidize a chamber or bed of nanosized powders. That is, abed may be expanded to more than double its original chamber or bedheight with hardly any elutriation of the nanoparticles.

In addition, the system and method of the present disclosureadvantageously provides for greater control of the fluidizing process,despite a high degree of mixing, thereby reducing powder loss relativeto conventional fluidized chambers or beds. In one aspect of the presentdisclosure, for example, once the chamber or bed has been expanded, thesupply of energy or force, e.g., vibration, may be terminated (orreduced) so that the chamber or bed remains expanded and fluidized for aconsiderable duration. Thus, the supply of energy or force in accordancewith one aspect of the present disclosure may advantageously only beutilized initially to aid in the break-up of interparticle forces andform nanoagglomerates, so that the chamber or bed can be effectivelyfluidized. Thereafter, such energy/force may be discontinued, as desiredby the system operator or applicable control systems.

Further, depending on the size distribution of the nanoagglomerates,some powder beds, under flow of fluidizing gas and external energysupply, e.g., vibration, may be divided into two distinct regimes, adense immobile phase and a smoothly fluidized mobile phase above thedense immobile phase. The dense immobile phase may be substantiallyeliminated according to the present disclosure by adding heavy permanentmagnetic particles to the mix, preferably near the dense immobile phase,and then exciting the magnetic particles via a magnetic field, e.g., anoscillating magnetic field. The use of an external force, e.g.,magnetic, acoustic, centrifugal/rotational and/or vibration excitationforces, advantageously provides for better control of the degree ofparticle movement. Combining such force(s) with fluidizing gas flowadvantageously achieves excellent mixing, smooth fluidization, and highbed expansion with very little particle loss in a safe and inexpensivemanner.

The systems and methods of the present disclosure are advantageouslywell suited for fluidization of nanoparticles (extreme Geldart Cpowders), utilizing one or more external forces and aeration (or a flowof another gas) to overcome fluidization difficulties often associatedwith cohesive particles (e.g., channeling, spouting, plug formation) andto thereby advantageously achieve vigorous fluidization in any of avariety of differently shaped fluidization chambers or beds (e.g.,tubular and/or rectangular fluidization beds).

According to further aspects of the present disclosure, fluidizationcharacteristics of a variety of different nanoparticles are provided andsuch fluidization characteristics are advantageously correlated withmacroscopic fluidization behavior (APF or ABF) of the nanoagglomerates.To establish such correlation, the properties of primary nanoparticleswere established in a conventional gravity-driven fluidized bed withoutany additional external forces present. In addition, a simple andeffective method for estimating the average size of agglomerates and bedvoidage around the agglomerates is provided. The estimation methodologycan then be used in models to determine the minimum fluidizationvelocity, pressure drop and other pertinent variables of thefluidization process, and to determine the external force(s) required toestablish a desired particle size distribution to achieve and supportefficacious nanoparticle/nanoagglomerate fluidization, as describedherein.

These and other advantageous features and functionalities of thedisclosed fluidization system and method/process for fluidizingnanoparticles will be apparent from the detailed description whichfollows, particularly when read in conjunction with the figures appendedhereto.

BRIEF DESCRIPTION OF THE DRAWINGS

To assist those having ordinary skill in the art to which the subjectmatter of the present disclosure appertains in making and using thedisclosed fluidization systems and methods/processes, reference is madeto the appended figures, wherein:

FIG. 1 is a plot of particle size distribution for a commerciallyavailable “as received” silica product;

FIG. 2 is a schematic drawing of an exemplary fluidization apparatus inaccordance with an illustrative aspect of the present disclosure;

FIG. 3 is a plot showing bed height as a function of time withvariations in aeration and vibration conditions according to anexemplary embodiment of the present disclosure;

FIGS. 4(a) and 4(b) are plots showing bed expansion ratios as a functionof time for different operating conditions according to an exemplaryembodiment of the present disclosure;

FIG. 5 is a plot showing pressure drop as a function of superficial airvelocity under specified operating conditions according to an exemplaryembodiment of the present disclosure;

FIGS. 6(a) and 6(b) are photographic images of fluidization performancewith and without the introduction of a magnetic field;

FIG. 7 is a plot showing bed expansion ratio and pressure drop forfluidization systems with and without magnetic excitation;

FIG. 8 is a plot showing bed expansion ratio and pressure drop forconventional fluidization of an 80/20 mixture before and after magneticprocessing;

FIG. 9 is a plot showing bed expansion ratio and pressure drop for“soft” agglomerates with and without magnetic excitation;

FIG. 10 is a plot showing bed expansion ratio and pressure drop for“hard” agglomerates with and without magnetic excitation;

FIG. 11 is a plot showing bed expansion ratio and pressure drop forconventional fluidization of“hard” agglomerates before and aftermagnetic processing;

FIG. 12 is a table showing minimum fluidization velocities for “soft”agglomerates, “hard” agglomerates, and an 80/20 mixture of hard/softagglomerates;

FIGS. 13(a) and 13(b) are plots of particle size distribution for “soft”agglomerates with and without magnetic field application;

FIGS. 14(a) and 14(b) are plots showing bed expansion and collapse for asoft agglomerate system with magnetic excitation according to anexemplary embodiment of the present disclosure;

FIG. 15 is a table showing minimum fluidization velocities and bedexpansion ratios for “soft” agglomerates with different mass ratios ofmagnets to nanoparticles;

FIG. 16 is a table showing minimum fluidization velocities and bedexpansion ratios for “soft” agglomerates with different intensities ofmagnetic field;

FIG. 17 is a table showing minimum fluidization velocities and bedexpansion ratios for “soft” agglomerates with different frequencies;

FIG. 18 provides a schematic diagram of an exemplary sound-assistedfluidization system according to the present disclosure;

FIGS. 19(a) and 19(b) provides images of bed behavior of SiO₂nanoparticle agglomerates with and without sound excitation,respectively;

FIG. 20 provides a plot of bed expansion relative to superficial airvelocity, with and without sound excitation, according to an exemplaryembodiment of the present disclosure;

FIG. 21 provides a plot of pressure drop relative to superficial airvelocity, with and without sound excitation, according to an exemplaryembodiment of the present disclosure;

FIG. 22 provides images of fluidization behavior at different soundfrequencies (300, 400, 500, 600 and 1000 Hz) according to an exemplaryembodiment of the present disclosure;

FIG. 23 provides a plot of dimensional bed height relative to soundfrequency according to an exemplary embodiment of the presentdisclosure;

FIG. 24 provides a plot of dimensional bed height relative to soundpressure level (dB) at two sound frequencies (100 and 400 Hz) accordingto an exemplary embodiment of the present disclosure;

FIGS. 25(a) and 25(b) provide schematic diagrams of an exemplaryrotating fluidized bed system according to the present disclosure;

FIG. 26 provides a plot of bed pressure drop relative to air velocityfor four (4) exemplary rotation speeds (indicated in terms of equivalentgravity force, in G) according to an exemplary embodiment of the presentdisclosure;

FIG. 27 provides a plot of bed height relative to air velocity for four(4) exemplary rotation speeds according to an exemplary embodiment ofthe present disclosure;

FIG. 28 provides a plot of pressure drop relative to air velocity forfour (4) exemplary rotation speeds according to an exemplary embodimentof the present disclosure;

FIG. 29 provides a further plot of bed height relative to air velocityfor four (4) exemplary rotation speeds according to an exemplaryembodiment of the present disclosure;

FIG. 30 provides a further plot of pressure drop relative to airvelocity for four (4) exemplary rotation speeds according to anexemplary embodiment of the present disclosure;

FIG. 31 provides an additional plot of bed height relative to airvelocity for four (4) exemplary rotation speeds according to anexemplary embodiment of the present disclosure;

FIG. 32 provides a plot of fluidization velocity relative to centrifugalforce for three exemplary powder systems according to the presentdisclosure;

FIG. 33 provides nanoparticle properties for a series of powders intabular form;

FIG. 34 provides fluidization characteristics of APF nanoparticles intabular form;

FIG. 35 provides fluidization characteristics of ABF nanoparticles intabular form;

FIG. 36 is an exemplary graphical representation of pressure drop andbed expansion data as a function of velocity in accordance with anillustrative aspect of the present disclosure;

FIGS. 37(a) and 37(b) are exemplary photographs showing a fluidizationbed with and without vibration, respectively, in accordance with anillustrative aspect of the present disclosure;

FIG. 38 provides a series of exemplary photographs showing a progressionof mixing during aerated vibrofluidization in accordance with anillustrative aspect of the present disclosure; and

FIG. 39 provides a series of exemplary photographs showing a progressionof mixing during magnetically assisted nanofluidization in accordancewith an illustrative aspect of the present disclosure.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENT(S)

1. Fluidization of Nanoparticles Using Vibrational and/or MagneticForces

According to an exemplary embodiment of the present disclosure,homogeneous fluidization of nanoparticles is advantageously achieved bycoupling aeration with vibration. Vibration (e.g., at frequencies in therange of 30 to 200 Hz, and vibrational acceleration in the range of 0 to5 g) has been found to achieve smooth fluidization of nanoparticles.Through introduction of vibrational energy, as described herein, thenanoparticle/nanoagglomerate particle size distribution isadvantageously modified to and maintained in a distribution range thatsupports and maintains efficacious fluidization. In exemplaryembodiments of the present disclosure, the minimum fluidization velocity(defined as the lowest gas velocity at which the pressure drop acrossthe bed reaches a plateau) has been measured at approximately 0.3-0.4cm/s, and been essentially independent of vibrational acceleration.Moreover, the bed expands almost immediately after the air isintroduced, reaching bed expansions of three (3) times the initial bedheight or higher. Hence, the bed appeared to exhibit a fluid-likebehavior at velocities much lower than the minimum fluidizationvelocity. According to such exemplary embodiments, fluidization ofnanoparticles is achieved due to the formation of stable, relativelylarge, and very porous agglomerates and bubbles/elutriation of particleswere essentially non-existent.

Referring to the drawings and, in particular, FIG. 2, an exemplarynanoparticle fluidization apparatus in accordance with an illustrativeaspect of the present invention is shown and generally represented byreference numeral 1. Apparatus 1 essentially has a gas supply source 2suitable for supplying a fluidizing gas, a vibration source 4 suitablefor providing a mechanical force, and a magnetic source 6 suitable forproviding a magnetic force. Other features that may preferably beassociated with the apparatus 1 include a gas inlet 8, a gauge 10 formeasuring gas flow, a distributor 12, fluidization bed chamber 14, avent 16 and/or an accelerometer 18.

In a preferred aspect of the present invention, apparatus 1 is wellsuited for aerated vibrofluidization. In this aspect of the invention,the fluidization bed chamber 14 may have a tube portion 15 of glass orany other suitable material, including, for example, metal, plastic, orceramic. The tube portion 15 has an inner diameter that may preferablyrange from at least about one (1) centimeter to several meters. Ifnecessary, electrostatic charge can be decreased, for example, via aDC-nozzle such as provided by Tantec, Inc. A DC-nozzle can achievestatic-neutralization by ionizing the air flowing through thefluidization bed chamber 14 by taking a voltage and transforming it intoa high voltage. (Input voltage: 120V AC+10-15%, 50/60 Hz, 3 groundplugs, Output Voltage: 12V DC, 300 mA). The fluidization bed chamber 14may be mounted on top of the vibration source 4. Optionally,acceleration may be measured by the accelerometer 18 (e.g., apiezoelectric accelerometer). Examples of fluidizing gases that may beutilized in conjunction with this and other aspects of the presentinvention include air, nitrogen, helium, oxygen, argon and/or othergases suitable for fluidized bed chamber reaction.

In an aspect of the invention, the flow rate of the fluidizing gas,which is preferably a dry compressed air, may be measured with the gauge10, such as, for example, by a rotameter or alternatively, any othersuitable flow measuring device. Depending on the application and size ofthe fluidization bed chamber 14 and the powder utilized, a typical flowrate may fall anywhere in the range of about a fraction of acentimeter/second to several centimeters/second. For instance, if thefluidization bed chamber 14 having a cylindrical bed of about 3 inchesis used with 12 nm silica particles (Aerosil® R972 silica), a velocityof 1 cm/sec. may preferably be employed to achieve vigorous fluidizationwith high bed expansion.

In another aspect of the invention, vibrational parameters (e.g.,frequency, amplitude, and vibrational acceleration), which may becontrolled by an inverter, for example, can be varied to achieve adesired effect on the degree of mixing and behavior of fluidization. Forinstance, suitable values for frequency might preferably range fromabout 20 to about 500 Hz; suitable values for amplitude preferably rangefrom about 0.001 to about 13.81 mm; and suitable values for accelerationmay preferably be as high as about 20 g's.

It follows from the foregoing that vibrational acceleration or intensitymay be defined as a ratio of vibration acceleration to gravityacceleration: r=(Aω²) /g where A=amplitude and ω=2 nf. Still further, asappropriate and/or needed, pressure drop may easily be measured by amanometer or a pressure transducer, for example, and recorded eithermanually or electronically via a computer. Operating efficacy may bemonitored and/or observed, as desired, by photographing the apparatus 1with a suitable camera, e.g., a digital camera to capture the behaviorof fluidization, such as smooth or bubbling.

In illustrative embodiments of the present disclosure wherein fluidizednanoparticles were generated by aeration and vibration, a fluidized bedconsisting of a glass tube with an inner diameter of 6.25 cm and heightof 35 cm was employed. The fluidized bed was equipped with a series ofports for sampling and pressure measurements. The distributor consistedof a porous sintered metal material. The bed was mounted on top of aLing Dynamic System vibrator, which can produce AC vertical sinusoidalwaves with accelerations up to 5.5 g (where g is the acceleration due togravity) measured by a piezoelectric accelerometer. The frequency (f) ofvibration could be varied from 30 to 200 Hz.

The powder used was Aerosil® R974 (Degussa) hydrophobic silica having aprimary particle size, particle density, bulk density, and externalsurface area of 12 nm, 2200 kg/m³, 30 kg/m³, and 200 m²/g, respectively.These silica nanoparticles were at the extreme end of Geldart's group Cpowder classification. Humidity is an important issue when dealing withpowders (especially hydrophilic powders) because of liquid bridges andelectrostatic effects. However, in the experiments described herein,hydrophobic silica was employed such that humidity did not play as largea role, and bone dry compressed air was used as the fluidizing gas. Theairflow rate was measured by a rotameter.

When the bed was in its typical mode of aeration (i.e., evidencingundesirable fluidization behavior, such as channeling, lifting as aplug, etc.), the vibration was turned on. Flow rate, pressure drop,vibrational acceleration, frequency and bed height measurements, as wellas visual observation of the fluidization behavior for each experiment,were all recorded. The vibration intensity is defined as the ratio ofvibrational acceleration to gravitational acceleration, Γ=(Aω²)/g, whereA is the amplitude of vibration and ω=27 πf. The pressure drop wasmeasured by a pressure transducer and recorded on a computer. Photoswere taken with a digital camera.

Two methods of sampling were employed, both of which yielded similarresults when the powder samples were viewed with a scanning electronmicroscope (SEM). The first involved aspirating out samples at differentheights of the bed through small openings along the side of the tube.These samples were then gently placed on SEM sample disks. The secondmethod consisted of gently dipping a SEM sample disk adhered with adouble-side carbon tape into the fluidized bed. The sample disk was thendirectly used for SEM analysis. In addition, to aid in viewingagglomerates in situ, an argon laser generator (Reliant 1000M,Laserphysics) with 3-watt power and a high-speed CCD camera with anextremely short exposure time were used. As described herein, it wasobserved experimentally that mechanical vibration helped break up thechanneling and spouting in a bed of nano-sized powders. Considerablysmaller gas velocities (but still much larger than that based on theprimary nanoparticle size) were adequate in these experiments, becausevibration provided sufficient energy to the system to overcomeinterparticle forces and form stable agglomerates.

Visual observation of a highly expanded bed revealed the presence of twodistinct layers: a small bottom layer consisting of very largeagglomerates and a larger top layer consisting of very smoothlyfluidized smaller agglomerates. SEM micrographs indicated that thefluidized agglomerates in the top layer ranged in size fromapproximately 5 to 50 μm. The bottom layer consisted of agglomeratesthat were measured to be as large as 2 mm. When the two layers wereseparated by aspirating out the smoothly fluidized agglomerates, andthis top portion was reused as the next bed, the bottom dense layer didnot reappear. This suggests that the dense layer simply contained thehard agglomerates, which were present in the as-received nanoparticles;such hard agglomerates could have formed during handling and storage.Under vibration, these large agglomerates would sink to the bottom ofthe bed since the vibration energy was not sufficient to break them upand the airflow was not large enough to fluidize them. In order to avoida large agglomerate size distribution, only the top portion of the bed(smooth layer) was used in all of our experiments described below.

A Beckman Coulter counter (dry module) was used to determine theagglomerate size distribution of the as-received silica powder.Representative Coulter counter results for pre-experiment powderindicated a mean agglomerate size of about 30-40 μm. This is highlysuspect since large agglomerates of size on the order of millimeters(perhaps formed during storage) could be observed visually. Thesecontradictory results suggest that the agglomerates are in general sofragile that any measurement method involving direct contact with thesample is not effective and reliable. It is believed that theagglomerates were broken up during the course of Coulter counter sizedistribution measurements, leading to agglomerate sizes of about 30-40μm.

As mentioned above, agglomerate samples were aspirated out of the bed atdifferent heights of the expanded fluidized bed and examined under SEM.The agglomerate sizes averaged approximately 30 μm. However, theagglomerates appeared very porous and fragile, and might have brokenduring their removal from the bed and/or during sample preparation forthe SEM. The agglomerate size estimated from pressure drop and bedheight data in fluidization experiments was considerably larger (˜160μm). Given the fragile nature of the agglomerates, it is reasonable toexpect that an equilibrium between agglomerate breakage and agglomerateformation is reached during the process of fluidization. Therefore, thetrue agglomerate size can only be found from measuring the agglomeratesdynamically as fluidization is occurring. The use of a high-speeddigital camera with an extremely short exposure time and a laser beammay be effective to estimate the dynamic agglomerate size in situ.

Although the bed was not initially (before application of vibration)fluidizable with aeration alone, the bed appeared to have a short-termmemory after vibration was applied. This memory effect was apparent inan experiment where the bed was first fully fluidized with vibration andaeration, and then was allowed to settle down by turning off thevibration and aeration. This settled bed could then be fluidized byaeration alone as long as it was done within a few minutes, which iscontrary to expectations given the Geldart Group C character of theprimary particles. Thus, once the bed was fluidized with introduction ofvibrational force, the interparticle networks in the original samplewere disrupted and the resulting agglomerates did not form strongcohesive networks for several minutes, even after the bed was allowed tosettle. However, if the bed was left longer than a few minutes in itsrest state, it became difficult to fluidize the bed.

Additionally, once the bed was fluidized with the aid of vibration andaeration, the vibration could be turned off and the bed would remainexpanded and fluidized for a considerable amount of time (approximately30 hours). FIG. 3 shows a comparison between the settling of a fullyexpanded bed after (a) aeration was left on and vibration was turnedoff, and (b) both aeration and vibration were turned off. Without bothvibration and aeration, the bed collapsed to its initial height withintwo (2) minutes. Based on these experimental observations, it appearsthat once the interparticle forces are disrupted, it takes a finite timeto return to the original undisturbed conditions.

In these experimental studies, the bed of nanoparticles, when aeratedwithout vibration, exhibited plug flow, channeling, and spouting. Whenairflow was coupled with sufficient vibration (so that Γ=(Aω²)/g>1), theimmobile bed would almost immediately begin to expand. Moreparticularly, the channels would close, the spouting would stop, and/orthe solid plug would break up. Increasing vibrational intensity, Γ,weakly affected bed height. The same phenomenon was also seen when onlythe top portion of the bed was used. At high vibration frequencies(f>100 Hz) and airflow rate, relatively large bubbles could be seen. Atlow frequencies (<50 Hz), many of the bubbles appeared to break anddissipate throughout the bed forming microbubbles (estimated to be about200 μm). Bubbles were not seen to coalesce, grow or break the uppersurface of the bed.

At modest fluidization gas velocities, the surface of the bed was verysmooth, there was no apparent disturbance from bubbles and practicallyno elutriation of particles was observed. At higher gas velocities (>2cm/s), the surface became unstable and elutriation of particles out ofthe tube could be observed. FIG. 4(a) shows bed expansion rate atdifferent Γ at a constant frequency of 50 Hz and constant superficialair velocity of 0.28 cm/s. In each experiment, the vibrationalparameters were first set at the desired conditions, and then theaeration was turned on (at time t=0) at the desired superficialvelocity. The steady state bed expansion increased with increasing Γ,but appeared to become independent of Γ at sufficiently large values ofΓ. In this series of experiments, the vibrational intensity was variedby changing the amplitude (A), while holding the frequency of vibrationconstant. This bed expansion behavior may be rationalized as follows: asthe vibrational intensity was increased, the size of the agglomeratedecreased at first and then became roughly independent of Γ, i.e.,reached a state of dynamic equilibrium.

The scaled acceleration Γ was not the only vibrational parameteraffecting steady state bed expansion. FIG. 4(b) illustrates that thesteady state bed expansion, at a constant superficial air velocity of0.28 cm/s, depended on the frequency of vibration, even when Γ wasmaintained constant; however, no systematic trend was manifest. It wasfound that at higher values of Γ, the effect of vibration frequency onthe steady state bed expansion decreased. It is clear from FIGS. 4(a)and 4(b) that at least two dimensionless groups involving A and ω wouldbe needed to capture the effect of vibration on fluidization behavior.

It is also clear from FIGS. 4(a) and 4(b) that the rate at which the bedexpanded depended on the vibrational parameters. The higher thefrequency or the lower the Γ, the slower the bed expanded. The rate ofbed expansion was roughly the same for Γ=4-6, but appreciably smaller atΓ=3 (see FIG. 4(a)). As seen in FIG. 4(b), the rates of bed expansion atfrequencies of 50, 70 and 100 Hz were comparable, while those at 30 and150 Hz suggest an inverse dependence on the frequency.

In all of the experiments performed, the measured pressure drop acrossthe bed at high gas velocities approximately equaled the weight of thebed per unit cross sectional area. FIG. 5 shows a typical set of resultsobtained in a vibrated fluidized bed of silica nanopowder, where boththe pressure drop across the bed and the bed expansion at increasing gasvelocities are presented. The pressure drop has been scaled with theactual measured weight of the bed per unit cross sectional area of thebed, while the bed height has been scaled with the height of the settledbed. It is clear from FIG. 5 that the pressure drop increased initiallywith gas velocity and then leveled off at high gas velocities. In theplateau region, the scaled pressure drop is very close to the expectedvalue of unity. A lower measured pressure drop than the weight of thebed could be due either to a loss of powder sticking to the wall, powderelutriation, or possibly to some non-uniformities in the gas flow due tothe relatively porous distributor that was used in the experiments. Onthe other hand, wall friction (Loezos et al., 2002) and cohesion betweenthe bed of particles with a layer of particles adhering tightly to thedistributor (Castellanos et al., 1999; Sundaresan, 2003) would result ina higher measured pressure drop than the weight of the bed. Our studieshave revealed only a weak effect of vibrational parameters on theconstant (plateau) pressure drop obtained at high gas velocities. Thus,there is no clear consensus on the effect of vibration on pressure dropacross the bed.

FIG. 5 also shows that bed expansion behavior in an exemplary systemaccording to the present disclosure was different than that observedwith Geldart group A particles where bed expansion begins only after theminimum fluidization velocity is exceeded. As soon as a vibrofluidizedbed (with Γ>1) was aerated, it began to expand even though the actualgas phase pressure drop was only a fraction of the bed weight per unitcross sectional area. As gas flow rate was increased, the bed continuedto expand and this was accompanied by a systematic increase in the gasphase pressure drop. The bed expansion continued into the constantpressure drop regime. The overall bed expansion could be in excess offive times the original height, and even at such dramatic bed expansionlevels the quality of fluidization appeared to be smooth.

Based on an agglomerate size of about 50 microns, the Reynolds number isless than 1. A number of studies (Mawatari et al., 2002; Noda et al.,1998; Tasirin et al., 2001; Erdesz et al., 1986) have found that, as thevibration intensity, Γ, is increased, the minimum fluidization velocityis decreased. Here, minimum fluidization velocity refers to the lowestgas velocity for which the pressure drop across the bed becomesconstant. However, in experiments according to the present disclosure(with Γ>1), frequency and other vibrational parameters had only a smalleffect on the minimum fluidization velocity, and this effect becameunobservable as Γ was increased.

In the exemplary experimental studies of the present disclosure, theminimum fluidization velocity (based on the definition above) wasdetermined to be around 0.3-0.4 cm/s (see FIG. 5). However, it is notedthat the bed exhibited fluid-like properties as soon as it started toexpand at velocities as low as 0.1 cm/s. Such a minimum fluidizationvelocity cannot be obtained empirically based on the primary particlesize, which demonstrates unequivocally that the disclosed system is onlyfluidizing agglomerates.

Since agglomerates are being fluidized according to exemplaryembodiments of the present disclosure, it is valuable to identify themanner in which voidage is defined. According to the present disclosure,voidage, ε, is defined as the fraction of the total bed volume occupiedby the fluid. Using 0.03 g/cm³ and 2.2 g/cm³ for the bulk density of asettled bed and primary nanoparticles density, respectively, it ispossible to calculate that ε˜0.9864. Thus, the bed of nanoparticles isalready highly fluffy even before fluidization. As the bed expands, εincreases to above 0.99. The agglomerate themselves were very porous.

In a further experimental study according to the present disclosure, asmall amount of silica was dyed blue with methylene blue formixing/tracer experiments. In such experimental study, the progressionof mixing of a small layer of blue particles placed at the top of thevibrated fluidized bed was observed. Within a few minutes, the entirebed turned blue and appeared well mixed, even though aeration wasapplied at a level well below the minimum fluidization conditions. Thisresult clearly showed that even in the region where the gas phasepressure drop was considerably smaller than the bed weight per unitcross sectional area, active mixing of the agglomerates occurred.

Preliminary SEM and EDX analyses showed that the agglomerates were wellmixed, at least on the agglomerate level, indicating thatvibrofluidization could be used to mix agglomerates of different speciesof nanoparticles. For example, nano-silica has been effectively mixedwith nano-titania and nano-molybdenum oxide. It is not known if theagglomerates retained their integrity during fluidization or if theybroke and formed again rapidly; in the former case, one would achievelittle mixing at the nano-scale. However, if mixing were indeed observedon a scale smaller than the agglomerate size, it would be indicative offragile agglomerates, which broke and formed repeatedly in avibrofluidized bed.

Thus, according to the preceding experimental studies, it has beendemonstrated that nanosized silica could be easily and smoothlyfluidized in the form of stable, very porous agglomerates withnegligible elutriation with the aid of vibration and aeration. Since thebed remained fluidized for a considerable amount of time with only airflow after vibration was turned off, vibration appeared to be necessaryonly initially to disrupt interparticle network and establish a desiredparticle size distribution, after which aeration was sufficient tosustain the bed in a fluidized and expanded state for an extended periodof time, i.e., a dynamic equilibrium was established. The mixing studiesdescribed above show that the application of vibrofluidization ofnanoparticles to mix different nanoparticles together to formnanocomposites also yields promising and advantageous results.

In another preferred aspect of the present invention, the apparatus 1may be well suited for aerated-magnetically assisted fluidization. Inthis aspect of the invention, the apparatus 1 may preferably besubstantially similar to that previously identified and/or described.However, the vibration source 4 may preferably be either replaced by orsupplemented with the magnetic source 6 preferably having one or moremagnetic elements or particles, such as, for example, barium-ferritepolyurethane coated magnets. Other magnetic particles may also be usedwhose sizes range from about 0.5 to about 5.0 mm. The magnetic source 6also preferably has one or more magnetic field generators preferablysurrounding a base portion 17 of the fluidization bed chamber 14.

In this aspect of the invention, the energy dissipated from thecollisions and/or spins of the magnetic particles due to interactionwith a magnetic field induced by the magnetic field generators may beutilized to facilitate effective fluidization of nanoparticles. Further,utilizing different loads of the magnetic particles may be an effectiveway to affect the energy inputted into the fluidization bed chamber 14.That is, the more magnetic particles used, the greater the energyprovided. The magnetic field may also be varied in order to change theenergy input. The magnetic field may, in one aspect of the invention, beinduced via a copper coil, for example, to induce an oscillatingmagnetic field strength of approximately 40 mT.

Typical magnetic particles may comprise hard barium ferrite (BaO.6Fe₂0₃), AlNiCo, rare earth metals, ceramics or various mixtures thereof.Such magnetic particles preferably have coercivities ranging from about200 to about 3000 oersteds. In order to minimize the attrition of themagnetic particles and the attrition by them on the container walls andscreen, it may be preferable to provide a soft coating over the magneticparticles. For example, the magnetic particles may be coated withpolymeric materials such as, for example, cured epoxy orpolytetrafluoethylene, to smooth the surface and make the magneticparticles more durable and resistant to wear. The magnetic particles mayalso be comprised of magnetic powder embedded in a polymeric matrix,such as barium ferrite embedded in sulfur cured nitrile rubber such asground pieces of PLASTIFORM™ Bonded Magnets, available from ArnoldEngineering Co., Norfolk, Nebr. The size of the magnetic particles mayvary from about ten times to about thousand times the size of the powdermaterial to be fluidized. The appropriate size of the magnetic particlesmay depend on and/or be based on the type of application, the density ofthe powder material, and/or the cohesive strength of the powdermaterial. The appropriate size of the magnetic particles may be readilydetermined by one skilled in the pertinent art. The shape of themagnetic particles may also vary, and may be spherical, elongated,irregular or other suitable shape.

The quantity of the magnetic particles required may be dependent on thequantity of the powder material to be moved, the bulk density of theparticular powder material, the cohesiveness of the particular powdermaterial, and/or environmental factors such as moisture, temperature, ortime of consolidation. Preferably, only that quantity of magneticparticles needed to cause the powder material near the container outletzone to move and/or flow may be used. In general, the weight of themagnetic particles should be approximately equal to the weight of thepowder material near the outlet zone, for example, if a conical bottomedhopper is used, the weight of the magnetic particles should beapproximately equal to the weight of the powder material in the lowerhalf of the conical section. However, the amount or weight of magneticmaterial may be less or more depending upon the nature of application.

The magnetic field generator(s) may preferably be supplied with power bymeans of oscillators, oscillator/amplifier combinations, solid-statepulsating devices and/or motor generators. A magnetic field maypreferably be generated by means of a solenoid coil, an air core orlaminated metal cores, and/or stator devices or the like. Further, thein a preferred aspect of the present invention, the magnetic fieldgenerator(s) may have one or more AC motor stators, i.e., motorspreferably with armatures removed, which may be powered by analternating current supply through variable output transformers. Inaddition, metal strips may be placed outside the magnetic fieldgenerator(s) to preferably confine the magnetic field to a specificvolume of space. The magnetic field preferably oscillates either bychanging the value in a sinusoidal fashion but keeping the direction thesame, or by changing the direction of the field itself, so that thefield rotates. The oscillating magnetic field can be caused, forexample, by using multiphase stators to create a rotating magneticfield, as disclosed in U.S. Pat. No. 3,848,363 to Loveness, or by usinga single phase magnetic field generator with an AC power supply at aspecified frequency to create a bipolar oscillating magnetic field. Inhighly cohesive powder materials, a rotating field is preferred becausethe magnetic particles do not have a possibility of not being moved dueto having an alignment with the direction of the field as in a bipolarfield.

A useful magnetic field is preferably one with an intensity sufficientto cause desirable motion and excitation of the magnetic particles, butnot large enough to demagnetize the magnetic character of magneticparticles that are moved by the oscillating magnetic fields. Themagnetic field intensity may range between about 1 oersted and about3000 oersteds, preferably between about 200 and about 2500 oersteds.

An important characteristic of the magnetic field may be defined by thefrequency of oscillations. The frequency of oscillations in theoscillating magnetic field affects the movement and subsequently thenumber of collisions that take place between a magnetic elementpreferably moved in the magnetic field and the surrounding powdermaterial/particles preferably caused to move and/or to be fluidized. Ifthe oscillating frequency is too low, the magnetic particles may movetoo slowly and may not have sufficient motion to cause the other powdermaterial to flow. If the oscillating frequency is too high, the magneticparticles may not be able to spin in the fast changing field due totheir inertia. The frequency may be from about 5 hertz to about 100,000hertz, preferably from about 50 hertz to about 1000 hertz, and even moreconveniently at the hertz which is commonly used in AC power supplies(i.e., 50 hertz, 60 hertz, and/or 400 hertz).

Having identified and/or described some of the various aspectsassociated with this exemplary aspect of the present invention,different methods of preparation and processing applications are nowdiscussed.

Depending on the bed chamber size, a measured amount of nano-sizedpowders may be carefully placed in the bed chamber 14 preferably abovethe distributor 12. The bed chamber 14 may of any suitable shape orconfiguration (e.g., tubular (3D) or rectangular (2D)) and maypreferably be placed vertically in operative association with theequipment (i.e., the vibration source 4 and/or the magnetic source 6).The distributor 12 may be made of several materials and take a varietyof different forms. For example, the distributor 12 may be a sinteredmetal disk, a ceramic porous plate, or simple wire meshes or clothes,all with apertures preferably small enough (usually less than about 40microns) to distribute the fluidizing medium as evenly as possible.Highly cohesive nanoparticles may not appreciably fall through thedistributor 12. The top of the bed chamber 14 may be sealed with a capand hose or tube, for example, leading to the vent 16 in case of anypowder elutriation, which may occur at relatively high velocities. Oncethe bed chamber 14 is operatively connected to the vibration source 4and/or the magnetic source 6, the vibration and/or magnetic field may beset at the desired settings (e.g., acceleration, frequency, etc.).Preferably, when the bed chamber 14 operatively cooperates with thevibration source 4 and/or with the magnetic source 6 at the desiredparameters, an air flow may be slowly turned on. One may verifyeffective fluidization using bed chamber expansion and pressuredifferentials. That is, not only does the bed chamber expand to providea good indicator of fluidization, but the pressure drop may also be agood indicator when it equals the weight of the bed chamber per unitarea. Pressure taps may be drilled into the bed chamber at variousdesired heights thereof so that pressure drops across different placesof the bed chamber may be obtained or quantified. One should rememberthat if the measured pressure drop includes the distributor 12, thepressure difference of the distributor 12 must be subtracted from thetotal pressure drop recorded to obtain the drop across the powder bed.If a dense layer forms at the bottom of the bed chamber 14 near thedistributor 12, the top portion of the powder bed may be elutriated orphysically taken out with an aspirator, for example, for later use inother applications (e.g., experiments without another dense layerforming). During successful fluidization in accordance with one or moreaspects of the present invention, samples may be taken for testing andanalysis with SEM, EDX, TEM, EELS, etc. Average size, an overall mappingof the composition, and/or the degree of mixing may be obtained usingsuch techniques. It is noted that if the powder material used isenergetic, different and appropriate means of analysis may be used andextra caution should be taken when using energetic materials forfluidization. For example, an electrostatic charge may be significantlydecreased using a DC nozzle that can ionize the fluidizing medium (e.g.,air). Nonetheless, energetic samples such as, for example, nano-aluminumand nano-sized molybdenum oxide (MIC) may also be fluidized and wellmixed in accordance with one or more aspects of the present invention.

This system is applicable at temperatures that range from about −100degree C. to about 2000 degree C. and pressures that range from about0.2 bars to about 2000 bars. The temperature and/or pressure may belimited mainly by the particular material being fluidized and/or thematerials used in constructing the apparatus 1. Preferably, ambienttemperatures and/or pressures (e.g., room temperature and/or atmosphericpressure) may be utilized. Humidity should preferably be regulated sothat moisture may be kept to a minimum. The presence of moisture mayaffect agglomeration of the powder material, although some humidity maybe helpful to minimize electrostatic charges. After the fluidizationprocess is completed, the powder material may preferably be collected ina clean container.

Additional experimental studies directed to the introduction of magneticforces using an oscillating AC magnetic field to excite relatively large(mm size) magnetized particles mixed in with nanoparticles agglomeratesto effect fluidization are further described herein. As demonstrated bysuch experimental studies, with the aid of an oscillating at lowfrequencies, the bed of nanoparticle agglomerates can be smoothlyfluidized, and the minimum fluidization velocity may be significantlyreduced. In addition, channeling or slugging of the bed disappears andthe bed expands uniformly without bubbles and with negligibleelutriation. The bed expansion and the minimum fluidization velocitydepend on the mass ratio of magnetic particles to nanoparticles, and theintensity and frequency of the oscillating magnetic field. The effectsof the intensity and frequency of the oscillating magnetic field and theweight ratio of magnets to non-magnetic nanoparticles are describedherein, as well as important fluidization parameters (such as theminimum fluidization velocity, pressure drop across the bed, and bedexpansion) in such systems are demonstrated. Unlike traditionalmagneto-fluidized beds, the magnetic particles used according to thepresent disclosure are permanent magnets, which furiously spin andcreate intense shear and agitations under an oscillating magnetic field.

The experimental system utilized herein consisted of a fluidized bed ofnanoparticle agglomerates, an oscillating electromagnetic field and avisualization apparatus. The fluidized bed was a vertical transparentcolumn with a distributor at the bottom. The column was a section ofacrylic pipe with an inner diameter of 57 mm and a height of 910 mm. Thedistributor was a sintered metal plate of stainless steel with athickness of 2 mm and pore size of 20 μm. To generate a uniform gasfield before the distributor, glass beads of diameter between 2.5 and3.5 mm were charged into a chamber placed below the distributor andabove the gas inlet to form a packed bed about 100 mm high. Anultra-fine mesh filter was located at the gas outlet to filter out anyelutriated nanoparticle agglomerates.

The fluidization behavior was visualized with the aid of a lightingdevice (Illumination Technologies, Model 150SX) and recorded by adigital camcorder (Sony, Digital 8). The magnetic particles were bariumferrite (BaO-6Fez03) coated with polyurethane (supplied by Aveka, USA),about 1.0-3.0 mm in size. These were permanent magnetic particles, whichwere recharged by contacting them with a strong permanent magnet beforeeach experiment and were then added to the bed of nanoparticles at aprescribed mass ratio. The shafts of two 1/20 HP electric motors (Dayton5M064B) were removed and the electromagnetic coils were placed oppositeone another around the lower part of the vertical transparent column bymounting them on an acrylic plate which holds the distributor. The coilswere driven by an alternating current generated by a power supply andwere capable of generating an oscillating magnetic field with anintensity up to 140 Gauss at the center of the coil. The power supply(Triathlon Precision AC Source) was rated to supply AC current withadjustable frequency and voltage. A strong cooling fan (Comair RotronTNE2A) was used to prevent the coils from overheating.

Fumed SiO₂ nanoparticles (Degussa Aerosil@ R974) with a primary particlesize of 12 nm and a bulk density of about 30 kg/m³ were used in theseexperimental studies. Due to surface treatment by the manufacturer, thenanoparticles were hydrophobic. Before the experiments, the particleswere sieved using a shaker (Octagon 2000) and a 35-mesh sieve opening(about 500 μm). The sieving process functioned as a “pre-treatment” stepwith respect to the nanoparticle feedstock and served to separate verylarge agglomerates, which may have been generated during packing,storage, and transportation. The selection of a mesh opening of 500 μmwas based on previous experimental findings that the typical size offluidized nanoparticle agglomerates is between 100 to 400 μm. The sizerange of the fluidized nanoparticle agglomerates was measured byanalyzing digital images of the fluidized agglomerates with the help ofa laser source (Laser Physics Reliant 1000m), a CCD camera (LaVisionFlowMaster 3S), and an image processing system (Dual Xeon CPU).

For purposes of the present disclosure, the smaller nanoagglomeratesthat pass through the openings of the 500 μm sieve are designated as“soft” and the larger agglomerates, from about 500 μm to more than 10 mmare designated as “hard”. These two different sized agglomerates and a“mixture” consisting of 80% soft agglomerates and 20% hard agglomeratesby weight (80/20) were selected to conduct the fluidization experimentsdescribed herein.

To minimize any effect of humidity on the fluidization experiments, purenitrogen from a compressed N₂ tank was used as the fluidizing gas. Thegas flow rate was measured and adjusted by two calibrated rotameters(Gilmont) with a combined flow rate range of up to 51.0 liters perminute. The pressure drop across the bed was measured with adifferential pressure transmitter (Cole-Parmer) with a measurement rangeof up to 1.0 inch of water; the lower pressure tap was placed slightlyabove the distributor (approximately 3 mm), so that it was not necessaryto measure the pressure drop across the distributor. A Gaussmeter(Walker Scientific Inc. MG-3A) with a range of from 1 to 10⁴ G was usedto measure the intensity of the oscillating magnetic field, which wasmeasured at the center point between the coils in the empty column(before charging the nanoparticles into the bed).

We have found that, even when using the same nanoparticles, if theexperiments are run with agglomerates of different sizes, the bed showsvery different fluidization behavior. For example, the soft R974agglomerates fluidize smoothly with large bed expansion (APF) at a lowminimum fluidization velocity of 0.23 cm/s. Here, we define the minimumfluidization velocity as the gas superficial velocity beyond which thebed pressure drop is no longer dependent upon the gas velocity andbecomes constant, and a relatively large bed expansion (typically 2 ormore times the initial bed height) occurs. A mixture consisting of 80%soft agglomerates and 20% hard agglomerates (80/20) also behaves as APF,but the minimum fluidization velocity is much higher (5.67 cm/s) thanthat of the soft agglomerates. However, the hard R974 agglomerates donot fluidize at all, even at a gas velocity as high as 13.2 cm/s. Atthis high gas velocity, significant particle elutriation was observed,and the fluidization experiment had to be interrupted to avoid largelosses of nanoparticles.

Typical fluidization behavior of the 80/20 mixture of SiO₂ nanoparticleagglomerates with and without the external oscillating magneticexcitation are shown in the photographic images of FIGS. 6(a) and 6(b),respectively. Without the external oscillating magnetic excitation, at asuperficial gas velocity of 0.65 cm/s (FIG. 6(a)), the nanoparticleagglomerates are first lifted as a plug and then the plug disintegratesto form undesirable, stable channels through which the gas passes; thebed expands slightly with an uneven surface and the pressure drop ismuch less than the bed weight, indicating that the nanoagglomerate bedis not fluidized.

However, if a sufficiently strong oscillating magnetic field is applied,the magnetic particles are set in motion (translation and rotation) andthe nanoparticle agglomerates are fragmented into smaller agglomeratesbecause of collisions with the magnets, the vessel wall, and thedistributor. After a few minutes, the particle size distribution of thenanoparticle agglomerates are brought into a desirable range, thechannels disappear, and the bed begins to expand slowly and uniformlyuntil it reaches its full expansion, of up to five (5) times the initialbed height. At the same time, the pressure drop reading is very close tothe weight of the bed, indicating fluidization of the entire bed. Ahomogenous fluidization state is established, as shown in FIG. 6(b), andthe surface is very smooth and even. After the experiment, the powderwas poured out and, from visual observation, most of the original largehard agglomerates are gone and the average agglomerate size appears verymuch smaller.

The pressure drop normalized with the bed weight per unit area and thebed expansion ratio as a function of superficial gas velocity throughthe bed are shown in FIG. 7 (with and without magnetic excitation). Asshown therein, solid lines reflect bed expansion ratios and dashed linesreflect pressure drops. The magnetic field intensity was 140G at thecenter of the field, and the mass ratio of magnets to nanoparticles was2:1 (with AC frequency of 60 Hz). With further reference to FIG. 7,U_(mf1) represents the minimum fluidization velocity without magneticexcitation, whereas U_(mf2) represents the minimum fluidization velocitywith magnetic excitation. It is clear from FIG. 7 that the magneticexcitation causes the bed to expand almost immediately as the velocityis increased and the bed fluidizes at a velocity more than one order ofmagnitude lower than that without magnetic assistance.

After separation from the magnetic particles, the nanoparticleagglomerates were recharged back into the column, and a secondfluidization experiment without magnetic assistance is conducted usingthese agglomerates. FIG. 8 is a comparison of the fluidizationcharacteristics of the 80/20 mixture, before and after magneticprocessing. Solid lines represent bed expansion ratios and dashed linesrepresent pressure drops. The magnetic field intensity was 140 G at thecenter of the field and the mass ratio of magnets to nanoparticles was2:1 (AC frequency of 60 Hz). A significant reduction in the minimumfluidization velocity from 5.67 cm/s (before magnetic “fragmentation”processing) to 1.25 cm/s (after magnetic “fragmentation” processing) isobserved, indicating that previous fluidization with magnetic assistancecauses the agglomerates to be fragmented into smaller ones and theaverage agglomerates size is reduced. However, the minimum fluidizationvelocity of these smaller agglomerates is still about an order ofmagnitude larger than the minimum fluidization velocity observed whenthe magnetic assistance is turned on.

The fluidization behavior of exemplary soft agglomerates is shown inFIG. 9. Solid lines represent bed expansion ratios and dashed linesrepresent pressure drops. The magnetic field intensity was 140 G at thecenter of the field and the mass ratio of magnets to nanoparticles was2:1 (AC frequency of 60 Hz). U_(mf1) represents the minimum fluidizationvelocity without magnetic excitation, whereas U_(mf2) represents theminimum fluidization velocity with magnetic excitation. The much smalleragglomerates fluidize well with and without magnetic excitation. In bothcases, the minimum fluidization velocities appear to be quite close toeach other, but at higher gas velocities (above minimum fluidizationvelocity), the bed expansion with magnetic assistance is higher thanthat without magnetic assistance. It is also noted that the ratio of themeasured pressure drop to the weight of the bed per unit area is belowunity for magnetic assisted fluidization. This may mean that some of thenanoagglomerates are not participating in the fluidization and may besticking to the magnets.

FIG. 10 shows the typical fluidization behavior (pressure drop and bedexpansion) of hard SiO₂ nanoparticle agglomerates (R974) with andwithout magnetic excitation. Solid lines represent bed expansion ratiosand dashed lines represent pressure drops. The magnetic field intensitywas 140 G at the center of the field and the mass ratio of magnets tonanoparticles was 2:1 (AC frequency of 60 Hz). U_(mf1) represents theminimum fluidization velocity without magnetic excitation, whereasU_(mf2) represents the minimum fluidization velocity with magneticexcitation. The size of the hard agglomerates was in a wide range, from0.5 mm to about 10 mm. Without the magnetic excitation, even atsuperficial gas velocity as high as 13.2 cm/s, the hard agglomeratescould not be fully fluidized. Visual observation reveals that thesmaller hard agglomerates are in motion at the top of the bed, but thelarger agglomerates remain at the bottom of the bed, causing the gas toflow in large channels between them. The bed showed almost no expansionand the pressure drop was much less than the bed weight, indicating thatthe bed was not fluidized.

After turning on the external magnetic field, however, the largeagglomerates become smaller and smaller due to fragmentation (disruptionof interparticle forces) caused by collisions with the magneticparticles, and these smaller agglomerates participate in the circulationof the bed. After a few minutes, the nanoparticle size distributionreaches a desired range and assumes a dynamic equilibrium. From thatpoint, even at the relatively low gas velocity of 0.94 cm/s, all of thelarge agglomerates disappear, and the bed expands slowly and uniformlyuntil it reaches full expansion, while the pressure drop reading is veryclose to the weight of the bed, indicating that the entire bed isfluidized.

The fragmentation caused by the magnetic processing is so obvious thatthe reduction in size of the hard agglomerates could be seen byinspection after the magnetic field and air flow were shut down. Uponremoving the magnetic particles, the nanoparticle agglomerates arerecharged into the chamber and a conventional fluidization experiment(no magnetic assistance) is performed. FIG. 11 is a comparison of thefluidization characteristics between the powder before and afterundergoing a magnetic assisted fluidization (fragmentation) processaccording to the present disclosure. Solid lines represent bed expansionratios and dashed lines represent pressure drops. The magnetic fieldintensity was 140 G at the center of the field and the mass ratio ofmagnets to nanoparticles was 2:1 (AC frequency of 60 Hz). U_(mf1)represents the minimum fluidization velocity before magneticfragmentation processing, whereas U_(mf2) represents the minimumfluidization velocity after magnetic fragmentation processing. A verylarge reduction in the minimum fluidization velocity (U_(mf)) fromgreater than 13.2 cm/s to 2.29 cm/s indicates that the averageagglomerates size has been significantly reduced through the magneticfragmentation processing.

The U_(mf) for the hard agglomerates after magnetic processing is 2.29cm/s, which is larger than the U_(mf) of 1.25 cm/s for the 80/20mixture, and also much larger than the U_(mf) of 0.23 cm/s for the softagglomerates. This indicates that the average size of hard agglomeratesand of the mixture after the fragmentation process is still larger thanthat of the soft agglomerates. Hence, in order to only investigate theeffect of magnetic excitation (e.g., magnet to nanoparticle mass ratios,AC frequencies, and different magnetic field intensities), and tominimize the influence of non-uniformity of the initial agglomerate sizedistribution, the soft agglomerates represent a good choice to conductthe comparison experiments.

At low gas velocities, conventional fluidization (no magneticassistance) of soft agglomerates or of the 80/20 agglomerate mixture,produced only slugging and channeling, while at sufficiently high gasvelocities, the bed can be fluidized smoothly. If the gas velocity isincreased above a certain level, bubbles can be observed in thefluidized bed. Fluidization of nanoparticle agglomerates occurs due tothe disruption of interparticle forces by the large hydrodynamic forcesgenerated at high gas velocities. However, for conventional fluidizationof hard agglomerates, even at a very high gas velocity, the bed couldnot be fully fluidized.

The mechanism of fluidization with the assistance of an oscillatingmagnetic field is two-fold: (1) fragmentation of large agglomerates intosmaller ones, and (2) transferring kinetic energy generated by theoscillating magnetic excitation to the nanoparticle agglomerates due tocollisions to disrupt the large interparticle forces between them. Thetable of FIG. 12 presents a summary of the minimum fluidizationvelocities for the soft, hard and 80/20 agglomerate mixture. For thesoft agglomerates, magnetic excitation has little effect, but itproduces a definite improvement in fluidization behavior for the 80/20mixture. Even for the hard agglomerates, magnetic excitation changes thefluidization characteristics significantly, from no fluidization tosmooth, bubble-less, agglomerate particulate fluidization (APF) withvery large bed expansion up to five (5) times the initial bed height.

The minimum fluidization velocity is also significantly reduced fromhigher than 13.2 cm/s to 0.38 cm/s. Without magnetic excitation, at agas velocity of 13.2 cm/s or higher, extremely strong elutriation couldbe observed, while with magnetic excitation, at the low gas velocity of0.38 cm/s, elutriation was negligible. The substantial reduction in theminimum fluidization velocity resulting in smooth and bubble-lessfluidization with little elutriation offers significant benefits forindustrial applications where good mixing and high rates of heat andmass transfer with little gas by-passing are required.

Moreover, optical measurements demonstrate that the mean agglomeratesize of the decreases by roughly 100 μm during magnetic processing (frommean measurement of 315 μm to mean measurement of 196 μm). As shown inthe plots of FIGS. 13(a) and 13(b), the agglomerate size distribution isadvantageously shifted downwards through magnetic processing accordingto the present disclosure, establishing a dynamic equilibrium thatfacilitates effective bed fluidization. FIG. 13(a) reflects the particlesize distribution for a “soft” agglomerate system without magnetic fieldapplication (i.e., control) and FIG. 13(b) reflects particle sizedistribution with magnetic field application (140 G, 60 Hz, mass ratioof magnets to nanoparticles of 2:1). The data reflected in FIGS. 13(a)and 13(b) was generated through in situ optical measurements on thefluidized bed surface.

According to experimental observation, when the magnetic excitation isturned on (140 G, 60 Hz; 2:1 ratio of magnets to nanoparticles), thefluidization behavior of the nanoparticle bed does not changeimmediately, and it will take several minutes for the bed to beginexpanding. The fluidized bed does not reach full expansion for a periodof about 5 to 15 minutes, i.e., a state of dynamic equilibrium. The bedexpansion as a function of time for R974 silica at different gasvelocities is shown in FIG. 14(a); the higher the velocity, the quickerthe bed expansion. Similarly, when turning off the magnetic excitation,it also takes a short period of time, typically 10-30 seconds, for thebed to begin to collapse, and the collapse will last from 1 to 3 minutesbefore reverting back to a fixed bed with uneven surface. The bedcollapse as a function of time is shown in FIG. 14(b); the higher thegas velocity, the longer it will take for the bed to collapse.

Additional fluidization experiments with magnetic assistance (140 G, 60Hz) were conducted using soft agglomerates for four different massratios of magnets to nanoparticles, varying from 1:4 to 2:1. The tableof FIG. 15 presents the values of U_(mf) and the bed expansion ratios attwo different gas superficial velocities that were observed for thesefour cases. This table shows that the minimum fluidization velocity andbed expansion depends on the magnet to nanoparticles mass ratio, withU_(mf) decreasing from 1.61 cm/s to 0.26 cm/s as the mass ratioincreases from 1:4 to 2:1. This reduction indicates that adding moremagnetic particles to the bed results in more kinetic energy transportedfrom the magnets to the nanoagglomerates, causing more fragmentation andeasier fluidization. The results set forth in the table also show thatthere is little benefit in increasing the ratio of magnets tonanoparticles above 1:1. It is also noted that the minimum fluidizationvelocities for low mass ratios of magnets to nanoagglomerates areactually higher than were observed for the nanoagglomerates without anymagnetic assistance. This behavior is probably due to the additionaldrag of the gas on the magnetic particles.

The table of FIG. 16 presents the values of U_(mf) and bed expansionratio at a fixed superficial gas velocity for three different magneticfield intensities when fluidizing soft nanoagglomerates, keeping theratio of magnets to nanoparticles at 2:1. The center point of the columnaround which the 2 coils are placed was selected as the reference pointfor measuring the intensity of the magnetic field and it was observedthat, when using a magnetic field intensity of less than 80 G, the bedcould not be fluidized. Hence, three (3) different intensities (100,120, and 140 G) were selected to conduct the fluidization experiments.As shown in FIG. 16, the minimum fluidization velocity is a strongfunction of the magnetic field intensity and U_(mf) and decreasesrapidly as the intensity of the magnetic field increases, indicatingbetter fluidization. The values of the bed expansion are quite close toone another, but they are nonetheless consistent with the trend that thebed will expand more in a stronger magnetic field.

The table of FIG. 17 presents the values of U_(mf) and bed expansionratio at a fixed superficial gas velocity for three (3) differentfrequencies of AC power, keeping the mass ratio of magnets tonanoparticles at 2:1 and the magnetic field intensity at 120 G at thecenter of the field. The table shows that the frequency of the magneticfield can significantly affect the minimum fluidization velocity. At thelower frequencies, i.e., 45 Hz and 60 Hz, the beds show similarfluidization behavior, and can be fluidized easily at a U_(mf) of 0.65cm/s and 0.51 cm/s, respectively. But at higher frequency, i.e., 80 Hz,the bed is difficult to fluidize, U_(mf) is as high as 2.64 cm/s, andthe bed expansion is much smaller than at the lower frequencies. At afrequency higher than 90 Hz, the bed could not be fluidized at all.

This foregoing experimental studies have shown that silica nanoparticleagglomerates can be easily and smoothly fluidized with the assistance ofmagnetic particles in an oscillating magnetic field. Due to asignificant reduction in the minimum fluidization velocity with magneticassistance, both elutriation of nanoparticle agglomerates and gas bypassin the form of bubbles is greatly reduced. With magnetic excitation,hard (larger than 500 μm) agglomerates change their fluidization patternfrom no fluidization to agglomerate particulate fluidization (APF) withlarge bed expansion. The minimum fluidization velocity of an 80% soft(smaller than 500 μm) and 20% hard agglomerate (80/20) mixture can alsobe significantly reduced. Magnetic-assisted nanoparticles fluidizationis easier to achieve and yields more uniform fluidization, and suchapproach can be used for “as-received powders”, i.e., straight out ofthe bag, without any pre-processing, and hence is very useful forpractical applications. Overall, the introduction of the magnetic energyaccording to the present disclosure significantly alters agglomeratesize, reducing it to achieve a desired size distribution, and allowingfor advantageous fluidization performance results.

The fluidization of nanoparticles and/or nanoagglomerates in accordancewith one or more aspects of the present invention may have a greatimpact on the processing and manufacturing of nanostructured products.It is known that mechanical, electronic, catalytic, optical, and/orother properties of a material are significantly enhanced when made ofnanoparticle components. For example, copper preferably composed ofnanocrystalline copper may be 5 times harder than copper that iscomposed of micron-sized copper particles. Further, the mixing ofnanosized aluminum and molybdenum oxide to produce MIC, an energeticmaterial that may have a variety of important military applications. Ithas been ascertained that good mixing of the two components on thenanoscale, as provided by the present invention, is essential forobtaining a viable, highly energetic product.

There are also several coating applications fornanoparticles/nanoagglomerates which may be imminent with the presentinvention. For example, in an alternative aspect of the presentinvention, the apparatus of the present invention may be provided with aspray nozzle preferably located above the bed surface. The spray nozzleis preferably suitable to spray the surface, where the particles arecontinuously circulating throughout the bed. The spray nozzle maypreferably be sized to deliver an appropriate amount of material for adesired amount of coating. Due to the loose structure of theagglomerates, individual coating of primary particles may only beforthcoming.

2. Sound-Assisted Fluidization of Nanoparticle Agglomerates

According to the present disclosure, it has been found that, with theaid of sound wave excitation at low frequencies, a bed of nanoparticleagglomerates can be readily fluidized and the minimum fluidizationvelocity is significantly reduced. For example, in the case of anexemplary nanoparticle material, namely hydrophobic fumed silicananoparticles (Degussa Aerosil® R974 having a primary particle size of12 μm) in the form of large 100 to 400 μm agglomerates, the minimumfluidization velocity was decreased from 0.14 cm/s in the absence ofsound excitation to 0.054 cm/s with the assistance of sound waveexcitation. In addition, under the influence of sound, channeling orslugging of the bed quickly disappeared and the bed expanded uniformly.Within a certain range of the sound frequency, typically from 200 to 600Hz, bubbling fluidization occurred. Both the bed expansion and bubblecharacteristics have been determined to be strongly dependent on thesound frequency and sound pressure level. However sound has almost noimpact on fluidization when sound frequency is extremely high, e.g.,above 2000 Hz. A relatively high sound pressure level (such as 115 dB)is needed to initiate the fluidization at such high frequencies.

Thus, according to the present disclosure, sound waves areadvantageously employed for fluidization purposes, either alone or incombination with other external energy sources, to provide excitation tonanoparticles that is relatively inexpensive, affects the entireparticle bed, and does not require any physical contact between thesound generator and the nanoparticles. The advantageously disclosedsound-assisted fluidization of nanoparticle agglomerates and theirfluidization characteristics are not only different from those observedusing other fluidization methods for nanoparticle agglomerates, but arealso different from sound-assisted fluidization of micron or sub-micronsized particles. The effects of sound frequency and sound pressure levelon the fluidization behavior, such as the minimum fluidization velocity,bubbling regime, pressure drop across the bed, and bed expansion, arealso disclosed herein.

A schematic diagram of an exemplary sound-assisted fluidization system100 is shown in FIG. 18. The exemplary system 100 includes a fluidizedbed 102 containing nanoparticle agglomerates 104, a sound excitationdevice 106, and a visualization apparatus 108. The visualizationapparatus 108 is provided for the sole purpose of monitoring theactivities and/or behavior of the nanoparticles within fluidized bed102, and is not required for implementations wherein such monitoring isnot necessary or desirable. The exemplary fluidized bed 102 is avertical transparent column with a distributor 110 at the bottom. In anexemplary embodiment of the present disclosure, the column is fabricatedfrom a section of acrylic pipe with an inner diameter of 57 mm and aheight of 910 mm. The exemplary distributor 110 is a sintered metalplate of stainless steel with a thickness of 2 mm and pore size of 20μm. Ultra-fine mesh filters 112 are located at the gas outlet to filterout any elutriated nanoparticle agglomerates.

The disclosed sound excitation device 106 includes a 63 mm loudspeaker114 that is powered by a sound amplifier 116 that communicates with asignal generator 118. The loudspeaker 114 is installed on the top offluidized bed 102. A precision sound pressure level meter (not pictured)may be used to measure the sound pressure level. According to anexemplary embodiment of the present disclosure, sound excitation system106 is capable of generating a sound wave in fluidized bed 102 with asound pressure level up to 125 dB and the sound frequency from signalgenerator 118 is typically adjustable, e.g., within a range extendingfrom 10 to 2 MHz. The fluidization behavior of the nanoparticles isvisualized with the aid of a lighting device (not pictured) and isrecorded by a digital camcorder 120. The visual images may beadvantageously analyzed directly by a computer 122.

According to an experimental use of the system 100, synthetic silicondioxide nanoparticles (Degussa, R974) with a primary particle diameterof 12 nm and a primary density of 2560 kg/m³ were employed. Thedisclosed sound-assisted fluidization system is not limited to use withsilicon dioxide nanoparticles, however, but may be employed with avariety of nanoparticle materials finding application in a variety ofcommercial fields. Before use in exemplary system 100, the nanoparticleswere sieved using a shaker (Octagon 2000) and a sieve of Mesh No. 35(mesh opening, about 500 μm). The sieving process served to remove verylarge agglomerates which may have been generated during packing,storage, and transportation. The selection of a mesh opening of 500 μmreflects the fact that the typical size of fluidized nanoparticleagglomerates is between 100 to 400 μm (although the present disclosureis not limited to such particle size distributions). The bulk density ofthe sieved nanoparticle agglomerates was 33.8 kg/m³.

Due to surface treatment by the manufacturer, the silicon dioxidenanoparticles are hydrophobic. To minimize any potential effect ofhumidity on the nanoparticle fluidization, pure nitrogen from acompressed N₂ tank 122 was used as the fluidizing gas. The gas flow ratewas measured and adjusted by a calibrated rotameter 124. With the aid ofan inclined tube monometer 126, the pressure drop across the bed wasmeasured. By measuring the pressure in the manner schematically depictedin FIG. 18, the pressure drop across distributor 110 was excluded.

Typical bed behavior of SiO₂ nanoparticle agglomerates with and withoutsound excitation are shown in FIGS. 19(a) and 19(b), respectively. Thenanoparticle agglomerates were first lifted in a slugging mode and thenthe bed disintegrated to form stable channels. The bed only expandsslightly with an uneven surface, as shown in FIG. 19(a). Once asufficiently strong sound is applied, the instabilities in the bedcollapse in a couple of seconds, the channels disappear, and the bedexpands rapidly and uniformly until it reaches the full expansion. Ahomogenous fluidization state is easily established, as shown in FIG.19(b).

Typical fluidization characteristics, including the minimum fluidizationvelocities, bed expansions and bed pressure drops with and without soundexcitation, are illustrated in FIGS. 20 and 21, respectively. Anadvantageous substantial reduction in the minimum fluidization velocitywith the introduction of the disclosed sound energy is apparent. For thetest material, i.e., the DeGussa Aerosil® R974 nanoparticles, theminimum fluidization velocity was reduced from 0.14 cm/s in the absenceof sound energy to 0.054 cm/s with sound excitation. As used herein, theminimum fluidization velocity is defined as the gas superficial velocitybeyond which the bed pressure drop is no longer dependent upon the gasvelocity and becomes nearly constant.

As noted above, at low gas velocities, only the slugging and channelingoccur in a fluidized bed of nanoparticle agglomerates while, atsufficiently high gas velocities, the bed can be fluidized smoothly.Fluidization of nanoparticle agglomerates occurs due to the effectivebreakup of large agglomerate clusters by the large hydrodynamic forcesat high gas velocities. With the aid of sound excitation, however, thebreakup of large agglomerate clusters takes place due to a combinedeffect of hydrodynamic forces and acoustic excitations. Through theintroduction of such energy, the particle size distribution isadvantageously shifted downward, thereby facilitating efficient andefficacious fluidization according to the present disclosure.

FIG. 22 shows a series of representative snapshots of the fluidizing bedat different sound frequencies. At a fixed sound level output (e.g., 125dB in FIG. 22), the bed of nanoparticle agglomerates can only befluidized in a relatively narrow band of low sound frequency from 20 to1000 Hz. Furthermore, bubbles appear in an even narrower range, 200-600Hz, and as seen in FIG. 22, both the occurrence of bubbling and bubblesize are strongly dependent on the sound frequency. Due to therelatively high bed voidage observed when fluidizing nanoparticleagglomerates in the bubbling fluidization regime, the bubble size andthe bubble rising velocity can be easily identified using visualizationtechnology. The bed expansion is also strongly dependent on the soundfrequency, as seen in FIG. 23.

The effect of sound pressure level on the bed expansion is shown in FIG.24. It is noted that below a critical value of sound pressure level(e.g., 112 dB at 1000 Hz and 105 dB at 400 Hz in FIG. 24), there is nofluidization. The critical sound pressure level appears to be a functionof sound frequency. Within the range of the test conditions reflected inFIG. 24, the bed expansion increases monotonically as the sound pressurelevel increases.

Bed expansion and overall fluidization performance according to thisexemplary embodiment of the present disclosure are related, at least inpart, to the balance between the sound-assisted agglomerate breakup andthe sound-assisted agglomeration of the nanoparticles. The introductionof sound energy reduces agglomerate size and, once a desired agglomeratesize distribution, advantageous fluidization performance results. At lowfrequencies, the introduction of sound energy to a nanoparticle systemcontributes to a reduction in particle size distribution, therebyenhancing bed expansion and fluidization (e.g., up to frequencies ofabout 1000 Hz), as well as reductions in minimum fluidization velocities(e.g., R974 reduced from 0.2 cm/s to 0.05 cm/s; TiO₂ reduced from 5.17cm/s to 2.29 cm/s). In addition, at sound pressure levels of greaterthan about 90dB, fluidization behavior of nanoparticle systems isenhanced. The enhanced fluidization behavior achieved through soundenergy introduction supports or facilitates more uniform mixing, fastersurface reaction and/or better surface coating.

Based on the test results set forth herein, it is apparent thatnanoparticle agglomerates can be easily and smoothly fluidized with theassistance of sound energy at an appropriate sound pressure level andsound frequency. Since there is a significant reduction in the minimumfluidization velocity in the presence of sound, elutriation ofnanoparticle agglomerates is much reduced. The ability to fluidize theexemplary fumed silica nanoparticle agglomerates could only be achievedwithin a given range of sound frequency with a sound pressure levelabove a critical value. Bubbling fluidization occurs within an evensmaller range of sound frequency.

3. Fluidization of Nanoparticles and/or Nanoagglomerates in a RotatingFluidized Bed

According to a further aspect of the present disclosure, a rotatingfluidizing bed (RFB) system and associated method/process are providedfor use in advantageously fluidizingnanoparticles/nanopowders/nanoagglomerates. Use of the disclosedrotating fluidized bed system demonstrates a linear dependence betweenthe minimum fluidization velocity and the centrifugal force deliveredthereby. The centrifugal force is generally dependent on such factors asthe dimensions of the rotating system and the rotational speed thereof.For example, conditions may be selected whereby the rotating systemgenerates various force levels, e.g., forces that are 10, 20, 30 and 40times normal gravity force. Of note, it has been determined that onedisadvantage associated with fluidization of nanoparticles at normalgravity force is that high powder elutriation takes place at high gasvelocities; however, using a centrifugal force field, higher gas flowrates may be advantageously employed without having such high levels ofelutriation, while generating even smaller agglomerates sizes than inconventional fluidization.

Several factors can influence pressure drop across a rotating bedcontaining powders, such as elutriation, radial velocity, and overallunit design. It is further believed according to the present disclosurethat Coriolis forces and their effects should be considered as anadditional cause of pressure drop variations for rotating fluidized bedsystems. Indeed, it has previously been mentioned that rotation is anadditional factor for destabilization [Brouwers, Phase separation incentrifugal fields with emphasis on the rotational particle separator,Experimental Thermal and Fluid Science 26 (2002) 325-334], and thatrotation becomes important when the Reynolds number based on rotation ishigher than a certain value; secondary flows may also occur as aconsequence of the Coriolis force.

With reference to FIGS. 25(a) and 25(b), an exemplary rotating unit 200according to the present disclosure is schematically depicted. Rotatingunit 200 includes a chamber 202 that encloses a cylindrical porousstainless steel sintered mesh 204 with an aperture size of 20 μm, 2 mmof thickness, 400 mm of diameter and 100 mm of depth. Mesh 204 functionsto distribute the gas that passes through the bed, i.e., as a gasdistributor. This gas distributor turns along its axis of symmetry,moved by a motor 206 which is controlled by a speed variator.

Rotating unit 200 also includes a stationary cylindrical filter 208 of100 μm mesh with 2 mm of thickness, 100 mm of diameter and 90 mm ofdepth; the function of stationary filter 208 is to retain elutriatedfine powder. The covers of chamber 202 and mesh/gas distributor 204 aretypically fabricated of an appropriately rigid material, e.g., acrylicplastic. In an exemplary material, the covers are fabricated from atransparent or translucent material which allows the behavior of the bedinside the unit to be viewed.

Pressure taps 210 are placed between gas distributor 204 and the innerfilter mesh 208, as shown in FIGS. 25(a) and 25(b). The pressure dropacross the air distributor may be measured using a differential pressuretransmitter. The gas, e.g., air, delivered to the distributor may bemeasured by an area variable type flowmeter 212. Since it is generallynot possible to measure the bed pressure drop directly in a rotatingfluidized bed, the pressure drop across the air distributor mesh 204 maybe determined as a function of air velocity or flow rate before loadingrotating unit 200 with powder. Then the bed pressure drop can bequantified by subtracting the pressure drop measured when the unit isloaded with powder, and when the unit is empty.

Among other accessories, a digital camera may be associated withrotating unit 200 for use in recording the behavior of nanoparticleagglomerates during fluidization. In addition, a laser light may be usedto determine the expansion of the bed as well as the homogeneity of thebed's surface. Further; a vacuum system may be employed to removeexhaust from rotating unit 200 and the pressure transmitter may beadvantageously connected to a computer system for processing of datareceived therefrom.

Experiments have been conducted using a system that corresponds to thesystem schematically depicted in FIGS. 25(a) and 25(b). The powdersemployed in such experimental runs belonged to Geldart C classificationsince they are very fine and cohesive particles; however, some of thembehave like group “A” powders, specifically the APF behavior [Wang etal., Fluidization and agglomerate structure of SiO ₂ nanoparticles,Powder Technology, 124 (2002) 152-159], while others have bubblingfluidization, specifically, the ABF behavior as found in recentexperiments by our group, described later in this document. The testedpowders showed a strong cohesive behavior, but they were quite differentthan C powders due to their fluidization behavior and bulk density. Forpurposes of the noted experiments, the powders were sieved using ashaker and a sieve of Mesh No. 60 (mesh opening about 250 μm). Thissieving procedure was followed because it is believed that the largeagglomerates break the homogeneity of the flow field and make thefluidization more difficult. Fumed Silica Aerosil was employed having anapproximate tapped density of 50 g/l. The R974 material had an averageparticle size of 12 nm, while the R972 material had an average particlesize of 16 nm. In both cases, 70 grams were used; the bulk density ofthese powders was approximately 30 g/l. The tested titanium dioxide P25material had an average particle size of 21 nm, a tapped density of 130g/l, and a bulk density of about 90 g/l. A total of 250 grams were usedin the experiments and the initial bed height was close to 0.02 m.

The experimental steps can be summarized as follows. The unit wascleaned very carefully so as to ensure a uniform air field would begenerated by the air distributor. All of the component parts of therotating unit were assembled and all joints sealed in order to preventleaks. The presence of leaks would undesirably distort collectedpressure drop data. The pressure drop across the air distributor wasthen measured. For this purpose, the unit was run empty, and the airflow and the rotating speed were changed successively in order to findthe relationship between the distributor's pressure drop and the airflow.

Next, the test material was loaded into the unit and the rotating speedwas set at the desired value in order to increase the centrifugal force.Immediately thereafter, the air flow was increased slowly and relevantdata was recorded, i.e., air flow, pressure drop and bed height.Subsequently, the rotating speed was increased to higher values and thesame procedures were followed with respect to data collection.

FIG. 26 shows the measured air pressure drops at different values of airvelocity and at different rotating speeds translated in “G”s (i.e.,translated into gravity forces). The pressure drop increases until theminimum fluidization velocity is reached, then a constant pressure dropis observed. It is noted that the pressure drop does not uniformlymaintain a linear trend before reaching the minimum fluidizationvelocity (U_(mf)); it is believed that due to the centrifugal forceimparted by the rotating unit, the powder was compacted and thereforethe changing pressure is due to the irregularities of the bed beforereaching the fluidized state.

FIG. 27 shows the relative bed height as a function of air velocity forthe tested R974 material. It is noted that the compaction effect overthe bed that is effected by the centrifugal field changes the bulkdensity of the powder. Bed pressure drop data related to thefluidization behavior of R972 material is shown in FIG. 28. FIG. 29shows the relative bed height during fluidization of the R972 material.In the case of R972 material (as with the R974 material), there is acompaction effect over the powder as the centrifugal field increases. Itis believed that the centrifugal force is transmitted to all particlesin the bed by the particles that are in closer proximity to the airdistributor.

FIG. 30 shows the fluidization behavior for titanium dioxide P 25material. Of note, the amount of titanium dioxide loaded into the unitfor the experimental runs described herein was higher than the amount offumed silica because the bulk density of the titanium dioxide isapproximately three (3) times that of the silica; therefore, a largerpressure drop was expected due to the increase of the weight of the bedwithin the system. With reference to bed height response for thetitanium dioxide material and as shown in FIG. 31, there is not a largebed expansion as was experienced with the R974 and R972 silica powdermaterials. In addition, measurement of the increase in bed height wasvery difficult to achieve. This difficulty can be explained due to thehigher density (bulk and particle) of the titanium dioxide powder. Nosignificant elutriation was observed during the titanium dioxideexperimental runs.

Therefore, it can be concluded that fluidization behavior in therotating bed systems of the present disclosure differs based, at leastin part, on the characteristics of the particles processed in suchsystems. For the R974 and R972 fumed silica materials, the bed expansionbehavior can facilitate determination of the fully fluidized systemstate. By contrast, for the TiO₂ P25 material, the bed expansion waspoor and unstable, thereby giving no useful insight with respect to thefluidization state of the system.

FIG. 32 shows the relationship between minimum fluidization velocity andcentrifugal force for the three tested material systems. A lineardependence between the minimum fluidization velocity and centrifugalforce, as observed in prior studies and consistent with a model proposedby Kao et al. [Kao et al., On Partial Fluidization in Rotating FluidizedBeds, AIChE J. 33 (1987) 858].

The experimental pressure drop measurements can be affected by severalproblems during the experimental runs described herein, such as cloggingof the distributor, leaks across the distributor assembly, inaccuraciesin the readings of the flow rate, problems in the pressure readingsystem, etc. However, these systematic errors generally exhibit a levelof repeatability that can be determined and, therefore, actions can betaken to address the underlying problem(s). Preliminary analysis showsthat the theoretical predictions for the pressure drop in a rotatingfluidized bed only consider the effect due to the centrifugal forces,and do not account for the effects of the relative magnitude between theradial and tangential velocities and the gradient of the tangentialvelocity in the radial direction. Nanoparticles differ in this respectfrom micron and larger particles, because the radial air velocities fornanoparticles are much lower than those for the micron and larger sizedparticles. When such factors are all taken into account, one may findthat the current theoretical predictions such as Kao et al. [Kao et al.,On Partial Fluidization in Rotating Fluidized Beds, AIChE J. 33 (1987)858], may not be fully valid and may need to be corrected for othereffects, including but not limited to the Coriolis effects.

Based on the foregoing experimental data, the advantageous ability tofluidize nanoparticles, nanopowders and/or nanoagglomerates in arotating fluidized bed according to the present disclosure is clearlydemonstrated. The foregoing experimental data also shows that differentnanopowders exhibit different behaviors during fluidization in arotating bed. The advantages of the disclosed rotating fluidized bedrelative to conventional, non-fluidized systems include: lesselutriation of powder, higher air flow rate, higher powder load by unitarea of distributor, reduction of the size of agglomerates due to thehigher shear rate, and shorter processing time. The foregoing testresults further demonstrate a linear dependency of the minimumfluidization velocity and the artificial gravity force generated by thecentrifugal effect. In addition, it is believed that when fluidizingagglomerates of nanoparticles, effects such as the Coriolis forces andothers affect the pressure drop.

4. Gas Fluidization Characteristics of Nanoparticle Agglomerates

According to a further aspect of the present disclosure, the effects ofdifferent types of nanoparticles on gas fluidization characteristics ofnanoparticle agglomerates was determined. Taking advantage of theextremely high porosity of the bed, optical techniques were used tovisualize the flow behavior, as well as to measure the sizes of thefluidized nanoparticle agglomerates at the bed surface. Upon fluidizinga series of different nanoparticle materials, two types of nanoparticlefluidization behavior were observed, namely agglomerate particulatefluidization (APF) and agglomerate bubbling fluidization (ABF).

Highly porous nanoparticle agglomerates exhibit two distinctfluidization behaviors, APF (smooth fluidization without bubbles atminimum fluidization) and ABF (bubbles at minimum fluidization). APFagglomerates show very large bed expansions, up to five times theinitial bed height as the superficial gas velocity is raised, and theReynolds numbers for these nanoagglomerates at minimum fluidization arevery low (0.05 to 0.35), which indicate that the agglomerates are increeping flow. ABF nanoagglomerates fluidize with large bubbles and showvery little bed expansion as the superficial gas velocity is raised andthe Reynolds numbers at minimum fluidization are close to or higher than2.0, which indicate that hydrodynamic inertial effects cannot beneglected.

The difference in fluidization behavior between smooth, liquid like,bubble-less, particulate fluidization with high bed expansion (APF), andnon-homogeneous, bubbling, aggregative fluidization with low bedexpansion (ABF) has been found according to the present disclosure tolargely depend on the bulk density and the primary particle size of suchnanoparticles. Indeed, the fluidization of relatively small (less than20 nm) nanoparticles with a bulk density less than 100 kg/m³ appear tobehave as APF, whereas larger and heavier nanoparticles are more likelyto behave as ABF (see the Table included as FIG. 33 hereto).

On the basis of experimental data using classical fluidized particlessuch as FCC catalyst, UOP catalyst, and hollow resin, Romero andJohanson [Romero et al., Factors affecting fluidized bed quality, Chem.Eng. Progr. Symp. Series., 58 (38) (1958) 28-37] present a criterion tocharacterize the quality of fluidization as either smooth or bubbling,depending on the value of a combination of dimensionless groups. Thesedimensionless groups consist of the product (Π) of the particle to fluiddensity ratio, the Reynolds and Froude number (these are based oncalculated agglomerated properties, and not on primary particleproperties) at minimum fluidization, and the bed height to bed diameterratio: $\begin{matrix}\begin{matrix}{{\prod{= {{{Fr}_{mf}{Re}_{mf}\frac{\rho_{a} - \rho_{g}}{\rho_{g}}\frac{H_{mf}}{d_{t}}} < 100}}},\quad{{smooth}\quad{fluidization}}} \\{{\prod{= {{{Fr}_{mf}{Re}_{mf}\frac{\rho_{a} - \rho_{g}}{\rho_{g}}\frac{H_{mf}}{d_{t}}} > 100}}},\quad{{bubbling}\quad{fluidization}}}\end{matrix} & (1)\end{matrix}$wherein:

d_(t) diameter of vessel (chamber), cm

Fr_(mf) Froude number at minimum fluidization velocity,${{Fr}_{mf} = \frac{u_{mf}^{2}}{d_{a}g}},$dimensionless

H_(mf) bed height at minimum fluidization velocity, cm

Re_(mf) Reynolds number at minimum fluidization velocity, dimensionless

ρ_(a) density of agglomerate in fluidized bed, kg/m³

ρ_(g) density of gas, kg/m³

The porous nanoparticle agglomerates of the present disclosure behavedifferently than the classical solid particles used to obtain equation(1). Nonetheless, the values of the dimensionless groups (which aredesignated as “Π”) were calculated for a series of tested nanoparticlematerials. Unexpectedly and as shown in the Tables included as FIGS. 34and 35 herein, the calculated results agree remarkably well with thiscriterion of formula (1). For the eight APF nanoparticle materials setforth in the Tables of FIGS. 34 and 35, the values of Π are within therange of 0.008˜1.55 (which is much less than 100), whereas for the threeABF nanoparticle materials, the values of Π are within the range of398˜1441 (which is much larger than 100). Hence, the criteria set forthin formula (1) appear to be valid for nanoparticle agglomerates andtherefore provide a valuable tool or methodology for determining whethera nanoparticle of interest will behave as APF or ABF.

Thus, according to the present disclosure, a classification criterionbased on the value of a combination of dimensionless groups todifferentiate between particulate and bubbling fluidization forclassical solid fluidized particles may be advantageously employed topredict whether nanoparticles will behave as APF or ABF. Indeed,utilization of this criterion may be superior to using the size and bulkdensity of the nanoparticles to predict their fluidization behavior.

Moreover, it is demonstrated herein that fluidization of well-sievednanopowders may be effectively achieved in the absence of externalexcitation, e.g., external excitations based on vibration, magnets, etc.Thus, according to the present disclosure and without externalexcitation, nanopowders may be fluidized provided the nanoparticles arewell-sieved so that large, hard agglomerates are removed. Thisadvantageous result further enhances the flexibility and effectivenessof nanoparticle fluidization systems according to the presentdisclosure.

5. Combined Systems

According to the present disclosure, it is specifically contemplatedthat one or more of the energy modalities disclosed herein may beadvantageously employed either alone or in combination. Thus, forexample, the following energy source combinations may be employed toachieve advantageous fluidization of nanoparticles according to thepresent disclosure:

-   -   Vibratory force in combination with magnetic force;    -   Vibratory force in combination with sound energy;    -   Vibratory force in combination with a rotating fluidized bed;    -   Vibratory force in combination with at least two of magnetic        force, sound energy and a rotating fluidized bed;    -   Vibratory force in combination with magnetic force, sound energy        and a rotating fluidized bed;    -   Magnetic force in combination with sound energy;    -   Magnetic force in combination with a rotating fluidized bed;    -   Magnetic force in combination with at least two of vibratory        force, sound energy and a rotating fluidized bed;    -   Sound energy in combination with a rotating fluidized bed;    -   Sound energy in combination with at least two of vibratory        force, magnetic force and a rotating fluidized bed; and    -   A rotating fluidized bed in combination with at least two of        vibratory force, magnetic force and sound energy.

Thus, according to the present disclosure, systems and methods/processesfor fluidization of nanoparticles are provided that exhibit numerousadvantageous properties and results, including: less elutriation ofpowder, lower minimum fluidization velocities, in certain cases, higherair flow rate, higher powder load by unit area of distributor, reductionof the size of agglomerates due to the higher shear rate, improvedmass-transfer and shorter processing time. Moreover, in exemplaryimplementations of the present disclosure wherein multiple energysources are combined with a fluidizing gas source, e.g., combinations ofat least two ancillary energy sources selected from among vibratoryforces, magnetic forces, sound/acoustic forces, androtational/centrifugal forces, the application of such external energysources may be supplied at levels such that, in combination, theancillary energy supplied to the fluidization system affects the desirednanoparticle fluidization results. The disclosed systems andmethods/processes may also be employed with a variety of fluidizinggases, e.g., air, N₂, He, Ar, O₂ and/or combinations thereof. Thus, theability to supply multiple types and levels of energy providessignificant control and flexibility to the fluidization of nanoparticlesystems. The advantageous fluidization systems and methods/processesdisclosed herein may be used in processing a wide variety ofnanoparticle materials for use in various applications, includingapplications that involve the manufacture of drugs, cosmetics, foods,plastics, catalysts, energetic and bio materials, high-strength orcorrosion resistant materials, and in mechatronics andmicro-electro-mechanical systems. Effective dispersion of nanoparticlesis achieved according to the present disclosure, thereby facilitating ahost of nanoparticle-related processing regimens, e.g., mixing,transporting, surface property modifications (e.g., coating), and/ordownstream processing to form nano-composites.

The present disclosure having been thus described with particularreference to exemplary forms thereof, it will be readily apparent thatvarious changes and modifications may be made therein without departingfrom the spirit of the present disclosure as defined herein. Thefollowing additional examples are intended for illustrative purposesonly and should not be construed so as to limit or narrow the scope ofthe present invention in any way.

EXAMPLE 1

An apparatus as shown in FIG. 2 was used to fluidize nanopowders usingany gas such as air or nitrogen and vibration.

FIG. 36 shows an exemplary plot of observed pressure drop and bedexpansion vs. superficial air velocity. At gas velocities greater than0.1 cm/sec and a vertical sinusoidal vibration of 5.5 g's, the bedbegins to expand and continues to expand both before and after theminimum fluidization velocity, defined as the velocity at which thepressure drop across the bed is equal to the weight of the bed dividedby its cross sectional area. The bed expanded to four times its initialheight and appeared to be uniformly fluidized with negligibleelutriation.

EXAMPLE 2

Using the apparatus of FIG. 2, and 12 nm silica powders with a constantflow rate and vibrational parameters of 50 Hz and 2 g's, the silicapowders were fluidized.

FIGS. 37(a) and 37(b) illustratively show what may typically occurduring a fluidization process. With air or vibration alone, nothinguseful occurs to a conventional nanoparticle powder bed. When the twoare coupled together, however, the nanoparticle size distribution isreduced/lowered and the powder bed expands with vigorous particlemovement.

EXAMPLE 3

Using the apparatus of FIG. 2, and 12 nm silica, tracer silica dyed withmethylene blue and constant flow rate of dry air and vibrationalparameters of 50 Hz and 4 g's, was fluidized.

FIG. 38 shows the progression of mixing 12 nm silica with a small amountof the same nano-sized silica dyed with methylene blue. The bed wasoperated at a constant air velocity of 0.45 cm/see with a verticalsinusoidal vibration of 4 g's at a frequency of 50 Hz. As can be seen inthe figure, as soon as the vibration was turned on the bed started toexpand and uniform bubble less fluidization was observed. Within 2minutes, the entire bed turned blue, indicating not only goodfluidization, but also very good mixing.

EXAMPLE 4

Using the apparatus of FIG. 2, and 12 nm silica, carbon blacknanoparticles and constant flow rate of dry air and vibrationalparameters of 50 Hz and 4 g's, the silica was fluidized.

Similar results, depicted in FIG. 39, were obtained with magneticassistance instead of vibration. In this particular example, the weightof the magnets, whose size range from 1.4 to 1.6 mm, was double that ofthe silica bed. A small amount of carbon black, another nanosizedpowder, was used as the tracer and placed on top of the silica bed atthe start of the experiment. Again, within minutes, the entire bedshowed very good and complete mixing.

PROPHETIC EXAMPLE 1

Using the apparatus of FIG. 2 along with vibration and magneticexcitations, a coated nano-powder mixture of pigment and polymericmaterial may be fluidized for powder coating application. Metallicobjects to be coated may be heated to temperatures above the meltingtemperature of the polymer and dipped in the fluidized bed for an amountof time dependent upon the coating thickness desired. Very uniform, thincoatings may be achieved after processing.

Although the present disclosure has been described with reference toexemplary embodiments thereof, the present disclosure is not to belimited to such exemplary embodiments. Rather, it is contemplated thatmodifications, enhancements and/or variations to the disclosedfluidization systems and methods/processes may be made without departingfrom the spirit or scope of the present invention.

1. A method for fluidizing nanoparticles comprising the steps of: (a)providing a nanoparticle feedstock having an initial agglomerate sizedistribution; (b) exposing said nanoparticle feedstock to a flow offluidizing gas and at least one additional force or a pre-treatmentselected from the group consisting of (i) sieving, (ii) a vibrationforce; (iii) a magnetic force, (iv) an acoustic force, (v) a rotationalforce, and (vi) a combination of two or more of said forces; whereinexposure of said nanoparticle feedstock to said flow of fluidizing gasand said at least one additional force is effective to modify saidinitial agglomerate size distribution from said initial agglomerate sizedistribution to a second, reduced agglomerate size distribution; and (c)establishing an expanded fluidized bed with said nanoparticle feedstockin a substantially fluidized state, wherein the agglomerate sizedistribution of said nanoparticle feedstock in said fluidized state isin dynamic equilibrium and is substantially equivalent to said second,reduced agglomerate size distribution.
 2. The method of claim 1, whereinsaid fluidizing gas is selected from the group consisting of: air,nitrogen, helium, argon, oxygen and mixtures thereof.
 3. The method ofclaim 1, wherein said nanoparticle feedstock in said fluidized stateforms highly porous agglomerates in a size range of about 50 microns toabout 1000 microns.
 4. The method of claim 1, further comprising apre-screening step wherein said nanoparticle feedstock is sieved toremove nanoparticle agglomerates that exceed a predetermined thresholdsize.
 5. The method of claim 4, wherein said predetermined thresholdsize is about 500 μm.
 6. The method of claim 1, wherein said at leastone additional force is sufficient to disrupt interparticle forcesbetween nanoparticle agglomerates, thereby reducing the initial particlesize distribution of said nanoparticle feedstock.
 7. The method of claim6, wherein said at least one force is a magnetic force and said magneticforce is imparted by magnetic particles that are independent of saidnanoparticle feedstock.
 8. The method of claim 7, wherein said magneticparticles are not fluidized by said flow of fluidizing gas.
 9. Themethod of claim 7, wherein said magnetic particles are energized by aforce of at least 100 Gauss.
 10. The method of claim 6, wherein said atleast one force is a vibratory force.
 11. The method of claim 10,wherein said vibratory force is generated by vibrational energy of atleast 1.5 g.
 12. The method of claim 6, wherein said at least one forceis an acoustic force.
 13. The method of claim 12, wherein said acousticforce is generated by acoustic energy of at least 90 dB.
 14. The methodof claim 6, wherein said at least one force is a rotational force. 15.The method of claim 14, wherein said rotational force is generated bycentrifugal forces of at least 5 g.
 16. The method of claim 1, furthercomprising introducing a coating material such that said coatingmaterial coats said nanoparticle feedstock in said substantiallyfluidized state.
 17. The method of claim 1, wherein said nanoparticlefeedstock includes a first reactant, and further comprising introducingat least one additional reactant, such that a reaction occurs betweensaid first reactant and said at least one additional reactant when saidnanoparticle feedstock is in said substantially fluidized state.
 18. Themethod of claim 1, wherein said exposure of said nanoparticle feedstockto said flow of fluidizing gas and said at least one additional force orpre-treatment is effective to achieve at least one of the followingperformance attributes: a reduction in bubble level within the fluidizedsystem, a reduction in gas bypass relative to the fluidized bed, smoothfluidization behavior, a reduction in elutriation, a high level of bedexpansion, a reduction in gas velocity levels to achieve a desiredfluidization performance, enhanced control of agglomerate size oragglomerate distribution, and a combination of the foregoing performanceattributes.
 19. An apparatus for use in fluidizing a nanoparticlefeedstock, comprising: at least one gas inlet, at least one distributor,a fluidization chamber, and at least one vent; at least one ancillaryenergy source communicating with said fluidization chamber, said atleast one ancillary energy source effective to provide sufficient energyto a nanoparticle feedstock within said fluidization chamber to reducethe agglomerate size distribution of said nanoparticle feedstock by anamount effective to facilitate fluidization thereof, said at least oneancillary energy source being selected from the group consisting of: (i)a source of vibration force; (ii) a source of magnetic force, (iii) asource of acoustic force, and (iv) a source of rotational force.
 20. Theapparatus of claim 19, wherein said source of magnetic force is anelectric field generator coil operatively connected to one or moreelectric power supplies surrounding a portion of said fluidizationchamber.
 21. The apparatus of claim 19, wherein said fluidizationchamber has a substantially cylindrical geometry.
 22. The apparatus ofclaim 19, wherein said at least one ancillary energy source is a sourceof vibrational force.
 23. The apparatus of claim 22, wherein said sourceof vibrational force includes a mechanical, electromagnetic orpiezoelectric component that is caused to oscillate by an input current,voltage or drive signal from a power amplifier to impart saidvibrational force.
 24. The apparatus of claim 19, wherein said at leastone ancillary energy source is a source of magnetic force, and whereinsaid source of magnetic force includes at least one magnetic coiloperatively connected to one or more magnetic field generators.
 25. Theapparatus of claim 24, wherein said one or more magnetic fieldgenerators are positioned around at least a portion of said fluidizationchamber.
 26. The apparatus of claim 24, further comprising magneticparticles positioned within said fluidization chamber and wherein amagnetic field generated by said one or more magnetic field generatorscauses said magnetic particles to impart exciting force within saidfluidization chamber.
 27. The apparatus of claim 26, wherein saidmagnetic particles are substantially spherical and include a roughexterior surface.
 28. The apparatus of claim 24, wherein saidfluidization chamber defines a plurality of spaced stages, and whereinmagnetic particles are positioned within each of said plurality ofspaced stages.
 29. The apparatus of claim 28, wherein a first set ofmagnetic particles are confined to a first spaced stage and wherein asecond set of magnetic particles are confined to a second spaced stage.30. The apparatus of claim 19, wherein said at least one ancillaryenergy source is a source of acoustic force, and wherein said source ofacoustic force includes a function generator, an amplifier and at leastone loudspeaker.
 31. The apparatus of claim 19, wherein said at leastone ancillary energy source is a source of rotational force, and whereinsaid source of rotational force includes a motor for causing saidfluidization chamber to rotate around its axis or an angle offset fromsaid axis.
 32. The apparatus of claim 31, wherein said motor is adaptedto impart variable rotational speed to said fluidization chamber. 33.The apparatus of claim 19, wherein said at least one ancillary energysource is a source of vibrational force, and wherein said source ofvibrational force is positioned substantially below said fluidizationchamber and is adapted to impart axially oriented vibrations to saidfluidization chamber.
 34. The apparatus of claim 33, wherein said sourceof vibrational force is adapted to generate a vibrational force of atleast 1.5 g at a frequency of between about 20 Hz and about 200 Hz. 35.A method for mixing nanoparticles that comprises the steps of: (a)introducing a first nanoparticle species into a fluidization chamber;(b) introducing a second nanoparticle species into said fluidizationchamber; said first and second nanoparticle species to a flow offluidizing gas and at least one additional force or pre-treatmentselected from the group consisting of (i) sieving; (ii) a vibrationforce; (iii) a magnetic force, (iv) an acoustic force, (v) a rotationalforce; and (vi) a combination of two or more of said forces, (c)establishing an expanded fluidized bed with said first and secondnanoparticle species in a substantially fluidized state, wherein theagglomerate size distribution of said first and second nanoparticlespecies in said fluidized state is in dynamic equilibrium and issubstantially equivalent to a reduced agglomerate size distribution; (d)effecting mixing of said first and second nanoparticle species withinsaid substantially fluidized state.
 36. A method for treatingnanoparticles comprising the steps of: (a) providing a volume ofnanoparticles having an initial agglomerate size distribution; (b)introducing said volume of nanoparticles to a fluidization chamber (c)exposing said volume of nanoparticles to a flow of fluidizing gas and atleast one additional force or pre-treatment selected from the groupconsisting of (i) sieving, (ii) a vibration force; (iii) a magneticforce, (iv) an acoustic force, (v) a rotational force, and (vi) acombination of two or more of said forces; wherein exposure of saidvolume of nanop articles to said flow of fluidizing gas and said atleast one additional force or pre-treatment is effective to modify saidinitial agglomerate size distribution from said initial agglomerate sizedistribution to a second, reduced agglomerate size distribution; (d)establishing an expanded fluidized bed with said volume of nanoparticlesin a substantially fluidized state, wherein the agglomerate sizedistribution of said nanoparticles in said fluidized state is in dynamicequilibrium and is substantially equivalent to said second, reducedagglomerate size distribution; and (e) effecting a treatment of saidvolume of nanoparticles in said substantially fluidized state.
 37. Themethod of claim 36, wherein said treatment includes coating said volumeof nanoparticles with a coating material introduced to said fluidizationchamber.
 38. The method of claim 36, wherein said treatment includeseffecting a surface modification to said volume of nanoparticles in saidsubstantially fluidized state.
 39. The method of claim 36, wherein saidtreatment includes effecting a reaction between said volume ofnanoparticles and an additional reactant introduced to said fluidizationchamber.
 40. The method of claim 36, wherein said treatment includes achemical reaction, and wherein said volume of nanoparticles functions asa catalyst for said chemical reaction.